Keep room alias in the text field in the join dialog, if the room alias was not found

[vurpo]

If joining a room by its alias returns 404 (no such alias exists), that very likely is because of a mistyped room alias. It would make sense to keep the alias in the room join dialog (so it's still in the text field if the user reopens the dialog). This would make it easier to fix a mistyped alias.