Fix #1126: Check if album art file actually exists

[sampsyo]

Part of #1679 by @reiv.

This can be reproduced by simply deleting cover.jpg (or equivalent album art file) and then running beet fetchart. With this change, when invoking fetchart as a command, it will also check if the actual file given by artpath exists in that directory.

[sampsyo]

This one is obviously the most straightforward of the three, and a good idea regardless of the eventual diagnosis of #1679. A test would be nice, and we also need a changelog entry, but beyond that this is ready.

[reiv]

Added a simple test. Is this sufficient?

[sampsyo]

Looks perfect to me; thanks! Feel free to hit that green button if you like.

[sampsyo]

Lovely! Thanks again. ✨