Fix crash when subtitles are not found
More fixes on subtitles errors handling
Use report_error in subtitles error handling
Fix crash when subtitles are not found and the option --all-subs is given
Don't crash in FileDownloader if subtitles couldn't be found and errors are ignored
Merge pull request #764 from jaimeMF/subtitles_not_found

Fix crash when subtitles are not found