Return unfinished items to the queue when they can be retried.

data/queue.py

@@ -54,6 +54,10 @@ class WorkQueue(object):
   def complete(self, completed_item):
     completed_item.delete_instance()
 
+  def incomplete(self, incomplete_item):
+    incomplete_item.available = True
+    incomplete_item.save()
+
 
 image_diff_queue = WorkQueue('imagediff')
 dockerfile_build_queue = WorkQueue('dockerfilebuild')