Show error message if unable to protect media file

### Reproduction Steps

1. Upload a file into the media library and protect it for a course
2. Upload the same file (or different file with same filename) in the media library, and try to protect it

### Expected Behavior

Shows an error message that the file couldn't be protected

### Actual Behavior

Does not protect due to conflict (avoid overwriting the file in the protected folder), and does not show an error

### Tech Notes

Alternatively we could look to save the files (and all associated thumbnails) with a different filename, similar to how the media library appends `-1` etc to the filename on save.