KUriFilter: don't return an error on non-existing files.

Summary: This allows to use partially-typed paths for completion purposes.

Test Plan: Adjusted unittest passes.

Reviewers: emateli, elvisangelaccio, gregormi, dhaumann

Reviewed By: dhaumann

Subscribers: dhaumann, broulik, Frameworks

Differential Revision: https://phabricator.kde.org/D9217