Warn the user on missing file/folder Flatpak permissions

[nekohayo]

If I navigate with Nautilus to my ~/some_project/foo folder, in which I have some video file symlinked to another place in my XDG Videos folder, i.e. something like this: baz.webm -> /home/my_user/Vidéos/demos/baz.webm

...trying to play the baz.webm symlink from this folder, Clapper silently fails, it just shows the Clapper window with a black screen and no playback happening.

However, it turns out that it would be able to play that file, just that it didn't have the permissions to do so.
I had to go into flatseal and add additional filesystem permissions for ~/some_project:ro

This is not obvious except to the geekiest flatpak users.
Clapper should show an error dialog with a hint for the solution when this happens / when it is unable to open a file or folder.

[Rafostar]

With the ongoing code rework, currently this case would show:

We cannot really tell apart non-existing file from file we do not have read permissions for.