executeoperation forward error output to qwarning

  - with that we can see something in the verbose output

Change-Id: Ic427cfb93d04c1a4f6e3bc8d8355c5d6f49a8c3d
Reviewed-by: Niels Weber <niels.weber@digia.com>
Reviewed-by: Karsten Heimrich <karsten.heimrich@digia.com>