`KeyError` triggered by `ConnectionError: Connection closed by server`.

Hi maintainers, thank you for the package !

This line can fail with a ConnectionError, at least when using the Redis backend, possibly others too:

When that happens, queue is never added to self._subscribers[channel].

But the finally block will try to remove it nonetheless, causing a KeyError and hiding the original issue:

Possible solutions:

Using discard instead of remove
maybe moving the _backend.subscribe call outside the try block: when that call fails you can probably skip self._backend.unsubscribe. You'd still need to put a None in the queue though.

I could prepare a PR with any of those if they make sense.

encode / broadcaster