Make installer creation more robust

Do not exit if creating one offline (online) installer fails, just keep
on going generating the next ones.
In the end check if the /installer_output directory is empty. If empty
then we exit with error.

Change-Id: I00bfb183101aab4674bfc1f3d20c80182888fbee
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>
Improve Qt src package build handling

Change-Id: I7b6d91dd717487eaf55463540d0db2a167fc5d47
Reviewed-by: Sergio Ahumada <sergio.ahumada@digia.com>
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>