make sure unused media exist before deleting them
make sure unused media exist before deleting them

jakesyl/ruby-card,weihautin/anki,subfusc/anki,go38/anki,jakesyl/ruby-card,socialpercon/anki-1,abeyer/anki,sunclx/anki,holycrepe/anki,weihautin/anki,jkitching/anki,Stvad/anki,florianjacob/anki,eduOS/anki,Arthaey/anki,LucasCabello/anki,xuewenfei/anki,kenjhim/anki,hssm/anki,ospalh/libanki3,hssm/anki