Provide better UX for Clients when idle-timeout is reached

Currently, users who've connected to a scheduler that then times out don't get much information.

```python
In [1]: from distributed import Client

In [2]: client = Client("172.25.136.49:8786") # started with `dask-scheduler --idle-timeout=10s

In [3]: client
Out[3]:
# scheduler times out here <-----------------------------------
In [4]: distributed.client - ERROR - Failed to reconnect to scheduler after 10.00 seconds, closing client
_GatheringFuture exception was never retrieved
future: <_GatheringFuture finished exception=CancelledError()>
asyncio.exceptions.CancelledError
In [4]:
```

For reference, the traceback when using dask-gateway is a bit scarier

```pytb
distributed.client - ERROR - Failed to reconnect to scheduler after 10.00 seconds, closing client
_GatheringFuture exception was never retrieved
future: <_GatheringFuture finished exception=CancelledError()>
asyncio.exceptions.CancelledError
Exception in callback None()
handle:
Traceback (most recent call last):
File "/srv/conda/envs/notebook/lib/python3.8/site-packages/tornado/iostream.py", line 1391, in _do_ssl_handshake
self.socket.do_handshake()
File "/srv/conda/envs/notebook/lib/python3.8/ssl.py", line 1309, in do_handshake
self._sslobj.do_handshake()
ssl.SSLCertVerificationError: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: self signed certificate (_ssl.c:1124)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/srv/conda/envs/notebook/lib/python3.8/asyncio/events.py", line 81, in _run
self._context.run(self._callback, *self._args)
File "/srv/conda/envs/notebook/lib/python3.8/site-packages/tornado/platform/asyncio.py", line 189, in _handle_events
handler_func(fileobj, events)
File "/srv/conda/envs/notebook/lib/python3.8/site-packages/tornado/iostream.py", line 696, in _handle_events
self._handle_read()
File "/srv/conda/envs/notebook/lib/python3.8/site-packages/tornado/iostream.py", line 1478, in _handle_read
self._do_ssl_handshake()
File "/srv/conda/envs/notebook/lib/python3.8/site-packages/tornado/iostream.py", line 1409, in _do_ssl_handshake
return self.close(exc_info=err)
File "/srv/conda/envs/notebook/lib/python3.8/site-packages/tornado/iostream.py", line 611, in close
self._signal_closed()
File "/srv/conda/envs/notebook/lib/python3.8/site-packages/tornado/iostream.py", line 641, in _signal_closed
self._ssl_connect_future.exception()
asyncio.exceptions.CancelledError
Exception in callback None()
handle:
Traceback (most recent call last):
File "/srv/conda/envs/notebook/lib/python3.8/site-packages/tornado/iostream.py", line 1391, in _do_ssl_handshake
self.socket.do_handshake()
File "/srv/conda/envs/notebook/lib/python3.8/ssl.py", line 1309, in do_handshake
self._sslobj.do_handshake()
ssl.SSLCertVerificationError: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: self signed certificate (_ssl.c:1124)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/srv/conda/envs/notebook/lib/python3.8/asyncio/events.py", line 81, in _run
self._context.run(self._callback, *self._args)
File "/srv/conda/envs/notebook/lib/python3.8/site-packages/tornado/platform/asyncio.py", line 189, in _handle_events
handler_func(fileobj, events)
File "/srv/conda/envs/notebook/lib/python3.8/site-packages/tornado/iostream.py", line 696, in _handle_events
self._handle_read()
File "/srv/conda/envs/notebook/lib/python3.8/site-packages/tornado/iostream.py", line 1478, in _handle_read
self._do_ssl_handshake()
File "/srv/conda/envs/notebook/lib/python3.8/site-packages/tornado/iostream.py", line 1409, in _do_ssl_handshake
return self.close(exc_info=err)
File "/srv/conda/envs/notebook/lib/python3.8/site-packages/tornado/iostream.py", line 611, in close
self._signal_closed()
File "/srv/conda/envs/notebook/lib/python3.8/site-packages/tornado/iostream.py", line 641, in _signal_closed
self._ssl_connect_future.exception()
asyncio.exceptions.CancelledError
Exception in callback None()
handle:
Traceback (most recent call last):
File "/srv/conda/envs/notebook/lib/python3.8/site-packages/tornado/iostream.py", line 1391, in _do_ssl_handshake
self.socket.do_handshake()
File "/srv/conda/envs/notebook/lib/python3.8/ssl.py", line 1309, in do_handshake
self._sslobj.do_handshake()
ssl.SSLCertVerificationError: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: self signed certificate (_ssl.c:1124)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/srv/conda/envs/notebook/lib/python3.8/asyncio/events.py", line 81, in _run
self._context.run(self._callback, *self._args)
File "/srv/conda/envs/notebook/lib/python3.8/site-packages/tornado/platform/asyncio.py", line 189, in _handle_events
handler_func(fileobj, events)
File "/srv/conda/envs/notebook/lib/python3.8/site-packages/tornado/iostream.py", line 696, in _handle_events
self._handle_read()
File "/srv/conda/envs/notebook/lib/python3.8/site-packages/tornado/iostream.py", line 1478, in _handle_read
self._do_ssl_handshake()
File "/srv/conda/envs/notebook/lib/python3.8/site-packages/tornado/iostream.py", line 1409, in _do_ssl_handshake
return self.close(exc_info=err)
File "/srv/conda/envs/notebook/lib/python3.8/site-packages/tornado/iostream.py", line 611, in close
self._signal_closed()
File "/srv/conda/envs/notebook/lib/python3.8/site-packages/tornado/iostream.py", line 641, in _signal_closed
self._ssl_connect_future.exception()
asyncio.exceptions.CancelledError
Exception in callback None()
handle:
Traceback (most recent call last):
File "/srv/conda/envs/notebook/lib/python3.8/site-packages/tornado/iostream.py", line 1391, in _do_ssl_handshake
self.socket.do_handshake()
File "/srv/conda/envs/notebook/lib/python3.8/ssl.py", line 1309, in do_handshake
self._sslobj.do_handshake()
ssl.SSLEOFError: EOF occurred in violation of protocol (_ssl.c:1124)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/srv/conda/envs/notebook/lib/python3.8/asyncio/events.py", line 81, in _run
self._context.run(self._callback, *self._args)
File "/srv/conda/envs/notebook/lib/python3.8/site-packages/tornado/platform/asyncio.py", line 189, in _handle_events
handler_func(fileobj, events)
File "/srv/conda/envs/notebook/lib/python3.8/site-packages/tornado/iostream.py", line 696, in _handle_events
self._handle_read()
File "/srv/conda/envs/notebook/lib/python3.8/site-packages/tornado/iostream.py", line 1478, in _handle_read
self._do_ssl_handshake()
File "/srv/conda/envs/notebook/lib/python3.8/site-packages/tornado/iostream.py", line 1400, in _do_ssl_handshake
return self.close(exc_info=err)
File "/srv/conda/envs/notebook/lib/python3.8/site-packages/tornado/iostream.py", line 611, in close
self._signal_closed()
File "/srv/conda/envs/notebook/lib/python3.8/site-packages/tornado/iostream.py", line 641, in _signal_closed
self._ssl_connect_future.exception()
asyncio.exceptions.CancelledError
Exception in callback None()
handle:
Traceback (most recent call last):
File "/srv/conda/envs/notebook/lib/python3.8/site-packages/tornado/iostream.py", line 1391, in _do_ssl_handshake
self.socket.do_handshake()
File "/srv/conda/envs/notebook/lib/python3.8/ssl.py", line 1309, in do_handshake
self._sslobj.do_handshake()
ssl.SSLEOFError: EOF occurred in violation of protocol (_ssl.c:1124)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/srv/conda/envs/notebook/lib/python3.8/asyncio/events.py", line 81, in _run
self._context.run(self._callback, *self._args)
File "/srv/conda/envs/notebook/lib/python3.8/site-packages/tornado/platform/asyncio.py", line 189, in _handle_events
handler_func(fileobj, events)
File "/srv/conda/envs/notebook/lib/python3.8/site-packages/tornado/iostream.py", line 696, in _handle_events
self._handle_read()
File "/srv/conda/envs/notebook/lib/python3.8/site-packages/tornado/iostream.py", line 1478, in _handle_read
self._do_ssl_handshake()
File "/srv/conda/envs/notebook/lib/python3.8/site-packages/tornado/iostream.py", line 1400, in _do_ssl_handshake
return self.close(exc_info=err)
File "/srv/conda/envs/notebook/lib/python3.8/site-packages/tornado/iostream.py", line 611, in close
self._signal_closed()
File "/srv/conda/envs/notebook/lib/python3.8/site-packages/tornado/iostream.py", line 641, in _signal_closed
self._ssl_connect_future.exception()
asyncio.exceptions.CancelledError
```

I wonder how we can make this UX a bit nicer. It's a bit difficult since the scheduler is the one going away, but the client is where the user is looking. Tossing some ideas out:

1. scheduler notifies the client that it's timing out, Update the client's repr to include the fact that it's timed out.
2. scheduler notifies the client that it's timing out, Use whatever mechanism is printing that `_GatheringFuture exception` to immediately print that the scheduler has timed out.

cc @jacobtomlinson if you have thoughts on this.