fetchart: Add some error handling to prevent crashes
fetchart: Add some error handling to prevent crashes

Today I had some network problems regarding dbpedia.org, which made
beets crash because a requests.exceptions.ConnectionError was raised
("[Errno 113] No route to host"). This commits adds some error handling
around network requests to prevent further crashes in the future.

shamangeorge/beets,shamangeorge/beets,ibmibmibm/beets,shamangeorge/beets,SusannaMaria/beets,jackwilsdon/beets,ibmibmibm/beets,SusannaMaria/beets,SusannaMaria/beets,ibmibmibm/beets,jackwilsdon/beets,jackwilsdon/beets,SusannaMaria/beets,sampsyo/beets,beetbox/beets,ibmibmibm/beets,beetbox/beets,shamangeorge/beets,jackwilsdon/beets,beetbox/beets,sampsyo/beets,beetbox/beets,sampsyo/beets,sampsyo/beets