QtIFW

Don't print error messages in case correct sudo password is entered

Any stderr data printed by 'sudo' was always printed as part of an error
message box, even if sudo succeeded. This issue is triggered very easily
by entering a wrong root password first, and then the correct password
on the second try.

Let's just print stderr data in case we have some *and* in case 'sudo'
returns a non-zero return code.

Change-Id: Icbb2d31cdf5c4d5d20ed4200553e9bf7e2b5bedd
Reviewed-by: Karsten Heimrich <karsten.heimrich@theqtcompany.com>
Reviewed-by: Katja Marttila <katja.marttila@theqtcompany.com>