Erred futures are never descoped

This test hangs on the final while loop:

```python
import weakref

@gen_cluster(client=True)
async def test_descope_erred_future(c, s, a, b):
f = c.submit(lambda: 1 / 0, key="f")
ref = weakref.ref(f)
with pytest.raises(ZeroDivisionError):
await f
del f
while ref():
await asyncio.sleep(0.01)
```
Additionally, the task never disappears from ``s.tasks``.

Replacing ``with pytest.raises(ZeroDivisionError): await f`` with
```python
try:
await f
except ZeroDivisionError:
pass
```
does not change anything.

Replacing ``with pytest.raises(ZeroDivisionError): await f`` with ``await wait(f)`` makes the issue disappear.

#### Workaround
Calling ``f.release()`` does not solve the client-side leak, but it does remove the future (and all of its graph) from the scheduler and workers.

@sjperkins I believe this may be closely related with what you've been seeing on the scheduler side?