signaling: Improve error handling when joining a room

[fancycode]

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If joining a room fails, the error is currently not handled:

spreed/src/utils/signaling.js

Line 1229 in 25b153b

console.debug('Joined', data, token)

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.