Fix --checkupdates shows error dialog if network is down.

Task-number: QTIFW-431

We need to add a separate console window in case we run on windows,
though we can't use a gui application as the lib might popup message
boxes. (MessageBoxHandler checks against a running core application)

Change-Id: Idad8dff44c6c0bfb0bb45e8a0fb408b3db832c3c
Reviewed-by: Niels Weber <niels.weber@digia.com>
Reviewed-by: Kai Koehne <kai.koehne@digia.com>