signaling: Improve error handling when joining a room

[fancycode]

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If joining a room fails, the error is currently not handled: https://github.com/nextcloud/spreed/blob/25b153bf39f38ffa5ba290a3ab4c0d604dc44f5e/src/utils/signaling.js#L1229

The join request is not retried or the error otherwise handled, so the signaling server will close the session if it's an anonymous user and disconnect the websocket: https://github.com/strukturag/nextcloud-spreed-signaling/blob/98bf947ce36657b58b815c997805fe9cb7bf8471/hub.go#L664

This will trigger a reconnect in the client which tries to resume the (already deleted) session.

@danxuliu I noticed this while testing #12604 where errors while joining can happen much more often than normally.