Do not re-use previously generated thumbnails in case of error

Summary:
If thumbnail generation had failed for a certain image, the thumbnail from the previous successful run would have been mistakenly used.

BUG: 355590

Reviewers: Gwenview, ngraham

Reviewed By: Gwenview, ngraham

Subscribers: ngraham, Gwenview

Tags: Gwenview

Differential Revision: https://phabricator.kde.org/D21337