Better UX for MUC connection issues

It would be nice to have proper feedback in a MUC tab when the connection has failed or is taking a long time. As explained here, there is currently no feedback (except in debug logs) when the joined domain name does not exist.

Likewise, when the domain is not running an XMPP server, the tab just says "Not connected" forever, ignoring received <error type=cancel>.

In addition, it would be nice to have feedback before the connection actually fails, when it takes a long time (eg. more than 5 sec). Something like Connection problem? instead of Not connected would do the trick.

There were talks about maybe having the server send an <error type=continue> to have the client explicitly know that s2s is taking a long time, so it doesn't have to guess whether a response is queued or not.