Give plugin error dialog a parent and prevent it from blocking.

Task-number: QTCREATORBUG-10697
Change-Id: I492cf635b3468dbd9b759200902b3d98c1021b12
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>