[FEATURE] Allow no cover image

[sbeckeriv]

Dearest Maintainer,

Thank you for your work on this code. I found that I was missing cover art for some files. I edited the file locally to log but not error around line 494ish. I also had to check for the extra_cover_file data before the cp command line 797-ish.

My edits work for me locally and I dont think this should be common. A flag for allow no error on missing cover art might be nice to include. Maybe one can today but it was unclear to me.

Once again. Thank you for your work on this code.
Becker