Negative return values are interpreted as 'process crashed' on
Windows: QTBUG-28735 . This in turn means the installer framework
always shows an error dialog.

By using positive numbers even for error conditions the installer
can decide to ignore certain errors instead.

Change-Id: Ib5cdd461372ac13fe417feb6ff43a7424c159f68
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>