Fix exceptions raised by the diffs worker

Fixes #465

workers/diffsworker.py

@@ -23,6 +23,12 @@ class DiffsWorker(QueueWorker):
       msg = ('Image does not exist in database \'%s\' for repo \'%s/\'%s\'' %
              (image_id, namespace, repository))
       logger.warning(msg)
+    except IOError:
+      # This exception is unrecoverable, and the item should continue and be
+      # marked as complete.
+      msg = ("Data could not be retrieved for image %s under repo %s/%s" %
+             (image_id, namespace, repository))
+      logger.exception(msg)
 
     return True