Display error instead of silently failing when asked to create folder that already exists

Summary:
BUG: 400423
FIXED-IN: 5.54

Test Plan:

1. mkdir ~/test
2. In Dolphin, try to create ~/test: folder not created
3. In Dolphin, try to create ~/test/foo: folder created
4. In Dolphin, try to create ~/testo: folder created

Reviewers: Dolphin, elvisangelaccio, dfaure

Reviewed By: Dolphin, elvisangelaccio, dfaure

Subscribers: anthonyfieroni, dfaure, emateli, elvisangelaccio, Codezela, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D17617