fix that threads are failing silently

sys.exit() inside a thread fails silently -> so using again
os._exit() how it was inside the enableThreadedPrint method
before.

Change-Id: I106a8f9862ada4055267d6bbe246fdc1117c5b07
Reviewed-by: Kai Koehne <kai.koehne@theqtcompany.com>
Reviewed-by: Iikka Eklund <iikka.eklund@theqtcompany.com>