- Make the layer going over the estimated size raise an exception

- Add a heuristic for estimating the layer size if it is 0
- Add a method where we can add a custom defined map of image -> size

util/queuefile.py

@@ -22,7 +22,11 @@ class QueueFile(object):
       if result is None:
         self._done = True
         break
-      
+
+      if isinstance(result, Exception):
+        self._closed = True
+        raise result
+
       self._buffer += result
       self._total_size += len(result)