Delete by ArtistID and post processer fixes.

Commit 5be36b5 is a potential fix for #1550.

Commit d490091 fixes exception catching, and adds an extra step before the actual post processing happens. In this step, it checks for valid media files and if they can be written to (if needed). I hope this prevents unfinished operations due to exceptions.

Also, the last commit fixes the 'Hail Mary' approach.