Fix exitcode from kioexec when executable doesn't exist (and --tempfiles is set)

Test Plan:
kioexec --tempfiles does_not_exist /tmp/foo
Used to exit with code 0, now it exits with code -1.
Found by a unittest that will soon be pushed as part of D28020.

Reviewers: apol, davidedmundson, elvisangelaccio

Reviewed By: davidedmundson

Subscribers: ahmadsamir, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D28060