move_art: Remove missing album art

Prevents a fatal file system error if the album art has been moved or renamed
and the beets library hasn't been manually updated.

beets/library.py

@@ -1030,6 +1030,12 @@ def move_art(self, operation=MoveOperation.MOVE):
         if not old_art:
             return
 
+        if not os.path.exists(old_art):
+            log.error(u'album art {0} not found, removing',
+                      util.displayable_path(old_art))
+            self.artpath = None
+            return
+
         new_art = self.art_destination(old_art)
         if new_art == old_art:
             return