Avoid calling finished() after error()"

Summary:
By not checking the return of runJob(), the slave might call finished() after already having called error() in handleError().
See T7995 for futher information.

Reviewers: elvisangelaccio, dvratil

Reviewed By: elvisangelaccio

Subscribers: dvratil, KIO GDrive

Tags: KIO GDrive

Differential Revision: https://phabricator.kde.org/D22462