Move/copy job: skip stat'ing sources if the destination dir isn't writable

Summary: BUG: 141564

Test Plan: Unit test, but also copying to / in dolphin, to check what the error message looks like.

Reviewers: shubham, broulik, bruns, chinmoyr, ngraham

Reviewed By: ngraham

Subscribers: ngraham, chinmoyr, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D17632