Fix "Save As" in failed-save message box

Summary:
After saving a file failed and clicking {nav Save As} will lead to
an error message "Gwenview cannot save this kind of documents" in
some situations though saving should be possible.
This is caused by passing the wrong source URL for `saveAs()`.

FIXED-IN: 18.04.1

Test Plan:
1. Start Gwenview in View or Browse Mode
2. {nav File > Save As} or {key Ctrl Shift S}
3. Try to save in a folder you don't have write permission (e.g. Root)
4. Click {nav Save As} in the warning dialog
5. Try to save in a writable folder
6. Saving should not fail with "Cannot save this kind of documents"

Reviewers: #gwenview, rkflx

Reviewed By: #gwenview, rkflx

Subscribers: rkflx

Differential Revision: https://phabricator.kde.org/D12372