Created attachment 340929 [details]
mockup illustrating error handling when joining rooms

If we attempt to join a room but fail to do so we need to have some error handling in place. The error handling should enable the user to retry joining the room again later.

The attached mockup shows a GtkInfoBar containing the error message along with a "Retry" button. The GtkInfoBar is only shown for rooms that we failed to join.

The error should explain why joinng the room failed and there can be several reasons, the most interesting being:

Error.CHANNEL_FULL
The channel is full. 

Error.CHANNEL_BANNED
You have been banned from the room.

Error.CHANNEL_INVITE_ONLY
The channel is invite-only

Error.CHANNEL_KICKED
You've been kicked from a channel.

See also:
https://people.gnome.org/~gcampagna/docs/TelepathyGLib-0.12/TelepathyGLib.Error.html

As you can see in the Error class, there are more errors but they are too technical and could probably just go behind a generic "Polari encountered an error while attempting to join this room."