rebaseline_server: display as much info as possible if expected/actual image is missing

gm/rebaseline_server/imagediffdb.py

 #!/usr/bin/python
 
 """
 Copyright 2013 Google Inc.
 
 Use of this source code is governed by a BSD-style license that can be
 found in the LICENSE file.
 
 Calulate differences between image pairs, and store them in a database.
 """
@@ skipping 250 matching lines @@
     key = (expected_image_locator, actual_image_locator)
     if not key in self._diff_dict:
       try:
         new_diff_record = DiffRecord(
             self._storage_root,
             expected_image_url=expected_image_url,
             expected_image_locator=expected_image_locator,
             actual_image_url=actual_image_url,
             actual_image_locator=actual_image_locator)
       except Exception:
-        logging.exception('got exception while creating new DiffRecord')
-        return
+        # If we can't create a real DiffRecord for this (expected, actual) pair,
+        # store None and the UI will show whatever information we DO have.
+        # Fixes http://skbug.com/2368 .
+        logging.exception(
+            'got exception while creating a DiffRecord for '
+            'expected_image_url=%s , actual_image_url=%s; returning None' % (
+                expected_image_url, actual_image_url))
+        new_diff_record = None
       self._diff_dict[key] = new_diff_record
 
   def get_diff_record(self, expected_image_locator, actual_image_locator):
     """Returns the DiffRecord for this image pair.
 
     Raises a KeyError if we don't have a DiffRecord for this image pair.
     """
     key = (_sanitize_locator(expected_image_locator),
            _sanitize_locator(actual_image_locator))
     return self._diff_dict[key]
@@ skipping 153 matching lines @@
 
   Args:
     expected_image_locator: locator string pointing at expected image
     actual_image_locator: locator string pointing at actual image
 
   Returns: already-sanitized locator where the diffs between expected and
       actual images can be found
   """
   return "%s-vs-%s" % (_sanitize_locator(expected_image_locator),
                        _sanitize_locator(actual_image_locator))