Remove client on failure

[darabos]

Resolves #87. Without this I'm getting more and more exceptions as I accumulate more and more disconnected clients. E.g.:

    +---------------- 15 ----------------
    | Traceback (most recent call last):
    |   File "/Users/danieldarabos/lynxkite-2000/.venv/lib/python3.11/site-packages/pycrdt_websocket/asgi_server.py", line 33, in send
    |     await self._send(
    |   File "/Users/danieldarabos/lynxkite-2000/.venv/lib/python3.11/site-packages/starlette/_exception_handler.py", line 48, in sender
    |     await send(message)
    |   File "/Users/danieldarabos/lynxkite-2000/.venv/lib/python3.11/site-packages/uvicorn/protocols/websockets/websockets_impl.py", line 358, in asgi_send
    |     raise RuntimeError(msg % message_type)
    | RuntimeError: Unexpected ASGI message 'websocket.send', after sending 'websocket.close' or response already completed.
    +---------------- ... ----------------
    | and 3389 more exceptions
    +------------------------------------

[davidbrochart]

Thanks @darabos.
I see that we never remove clients from the room, so this change seems to be needed even when there is no failure.