Make the connection error dialog optional
The connection error dialog is not helpful for the user and is arguably not even informative.
It happens quite frequently that Gajim will momentarily lose connection for just a moment and will reconnect. If the network connection is lost for an extended period of time, all network activity and not just jabber, ceases. In either case, forcing the user to acknowledge the event by clicking on a dialog is disruptive and annoying.
Error notification should only be done at the user's request so it should be made an optional preference. A less disruptive option would be to display a notification popup similar to the Gmail notification. This would alert the user but would not disrupt their workflow if they choose to ignore the event.