Replace RuntimeError with WebSocketDisconnected

[ruitcatarino]

Summary

Change the exception raised if we attempt to use a disconnected WebSocket from RuntimeError to WebSocketDisconnected.
Discussed in #2762
Issue #2766

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

I haven't changed the documentation, if there is a need for it I will gladly change it.

[Kludex]

That's not really what I meant... The exception already exists.

[ruitcatarino]

Well, reading the code, WebSocketDisconnect seems to be called when you want to disconnect a socket, in these cases the socket is already disconnected. In my opinion, it would make more sense to create a new exception (WebSocketDisconnected). Tell me if you still want to use WebSocketDisconnect and if so I will make such changes.

[dym-ok]

The change suggested by @ruitcatarino looks meaningful to me.
It would allow precise handling of situations like the one mentioned in #2543.

What stands in the way of accepting this change, can we somehow help?

[ruitcatarino]

After the revival of this MR, which, to be honest, I have revisited sporadically in the hopes of getting an update, since my team and I at work are still affected by the original issue that led me to create this pull request, I realized that this newly created exception could also be used in the following cases: raise RuntimeError('WebSocket is not connected. Need to call "accept" first.') → raise WebSocketDisconnected('WebSocket is not connected. Need to call "accept" first.') in the methods receive_bytes, receive_text and receive_json.