Federation reader should fail more gracefully on a refused connection

This issue has been migrated from [#6412](https://github.com/matrix-org/synapse/issues/6412).

---

### Description

At the moment a stack trace is logged:

```
2019-11-25 22:34:49,018 - synapse.http.server - 109 - ERROR - PUT-1573 - Failed handle request via 'FederationSendServlet':
Traceback (most recent call last):
File "/root/syntest/synapse/synapse/http/server.py", line 77, in wrapped_request_handler
await h(self, request)
File "/root/syntest/synapse/synapse/http/server.py", line 326, in _async_render
callback_return = await callback_return
File "/root/syntest/synapse/synapse/federation/transport/server.py", line 324, in new_func
origin, content, request.args, *args, **kwargs
File "/root/syntest/synapse/synapse/federation/transport/server.py", line 407, in on_PUT
origin, transaction_data
File "/root/syntest/synapse/synapse/federation/federation_server.py", line 121, in on_incoming_transaction
origin, transaction, request_time
File "/root/syntest/synapse/synapse/federation/federation_server.py", line 253, in _handle_incoming_transaction
await self.received_edu(origin, edu.edu_type, edu.content)
File "/root/syntest/synapse/synapse/federation/federation_server.py", line 264, in received_edu
await self.registry.on_edu(edu_type, origin, content)
File "/root/syntest/synapse/synapse/federation/federation_server.py", line 826, in on_edu
return await self._send_edu(edu_type=edu_type, origin=origin, content=content)
twisted.internet.error.ConnectionRefusedError: Connection was refused by other side: 111: Connection refused.
```

### Version information

- **Homeserver**: using matrix-synapse 1.60RC2