fetchart: Check if album art file actually exists
fetchart: Check if album art file actually exists

Fixes #1126.

mosesfistos1/beetbox,m-urban/beets,parapente/beets,sampsyo/beets,Freso/beets,kareemallen/beets,swt30/beets,jcoady9/beets,diego-plan9/beets,jackwilsdon/beets,shamangeorge/beets,sampsyo/beets,SusannaMaria/beets,MyTunesFreeMusic/privacy-policy,mosesfistos1/beetbox,parapente/beets,ibmibmibm/beets,jcoady9/beets,jackwilsdon/beets,LordSputnik/beets,pkess/beets,lengtche/beets,jcoady9/beets,swt30/beets,artemutin/beets,m-urban/beets,pkess/beets,artemutin/beets,xsteadfastx/beets,swt30/beets,mosesfistos1/beetbox,jmwatte/beets,mosesfistos1/beetbox,jackwilsdon/beets,mried/beets,beetbox/beets,jcoady9/beets,beetbox/beets,Kraymer/beets,lengtche/beets,artemutin/beets,lengtche/beets,diego-plan9/beets,madmouser1/beets,pkess/beets,ibmibmibm/beets,kelvinhammond/beets,SusannaMaria/beets,LordSputnik/beets,xsteadfastx/beets,m-urban/beets,shamangeorge/beets,kareemallen/beets,jmwatte/beets,kelvinhammond/beets,Freso/beets,Freso/beets,kareemallen/beets,MyTunesFreeMusic/privacy-policy,Freso/beets,beetbox/beets,diego-plan9/beets,mried/beets,kelvinhammond/beets,shamangeorge/beets,jackwilsdon/beets,shamangeorge/beets,SusannaMaria/beets,ibmibmibm/beets,beetbox/beets,LordSputnik/beets,sampsyo/beets,LordSputnik/beets,kelvinhammond/beets,xsteadfastx/beets,xsteadfastx/beets,mried/beets,Kraymer/beets,mried/beets,madmouser1/beets,jmwatte/beets,swt30/beets,MyTunesFreeMusic/privacy-policy,SusannaMaria/beets,sampsyo/beets,MyTunesFreeMusic/privacy-policy,Kraymer/beets,artemutin/beets,pkess/beets,madmouser1/beets,kareemallen/beets,diego-plan9/beets,lengtche/beets,jmwatte/beets,madmouser1/beets,Kraymer/beets,m-urban/beets,ibmibmibm/beets,parapente/beets,parapente/beets