Summary:
Before this patch, when you forced Krita to load file in
unknown format, it showed uncancelled progress bar and
showed the file in Recent Documents.
It happened when you dragged and dropped file to the
welcome screen or import animation frames or use Open->File
or Open -> Recent Files.
Progress bars were visible only when some image was opened.
This commit cancels progress bars and make sure unloadable
files are not added to Recent Documents.

CCBUG:402625

Test Plan:
1. Create an unloadable file. Can be "touch a".
2. Load it using one of the ways mentioned above
3. After loading, there is an error message. Click ok.
4. If you used Import Animation Frames, look at the bottom
of Krita window.
- before: progress bar still "loading"
- now: no progress bar visible
5. If you used other option, create a new file or open
something else, and you'll see the same effect.
6. Look at Recent Documents.
- before: unloadable file added to list
- now: no unloadable file added to list
(beside the ones that were aleady there after testing)

Reviewers: #krita, rempt

Reviewed By: #krita, rempt

Tags: #krita

Differential Revision: https://phabricator.kde.org/D19611