rebaseline_server: better reporting for bad URL fetch

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 73 matching lines @@
       actual_images_subdir: the subdirectory actual images are stored in.
       image_suffix: the suffix of images.
     """
     expected_image_locator = _sanitize_locator(expected_image_locator)
     actual_image_locator = _sanitize_locator(actual_image_locator)
 
     # Download the expected/actual images, if we don't have them already.
     # TODO(rmistry): Add a parameter that makes _download_and_open_image raise
     # an exception if images are not found locally (instead of trying to
     # download them).
-    expected_image = _download_and_open_image(
-        os.path.join(storage_root, expected_images_subdir,
-                     str(expected_image_locator) + image_suffix),
-        expected_image_url)
-    actual_image = _download_and_open_image(
-        os.path.join(storage_root, actual_images_subdir,
-                     str(actual_image_locator) + image_suffix),
-        actual_image_url)
+    expected_image_file = os.path.join(
+        storage_root, expected_images_subdir,
+        str(expected_image_locator) + image_suffix)
+    actual_image_file = os.path.join(
+        storage_root, actual_images_subdir,
+        str(actual_image_locator) + image_suffix)
+    try:
+      expected_image = _download_and_open_image(
+          expected_image_file, expected_image_url)
+    except Exception:
+      logging.exception('unable to download expected_image_url %s to file %s' %
+                        (expected_image_url, expected_image_file))
+      raise
+    try:
+      actual_image = _download_and_open_image(
+          actual_image_file, actual_image_url)
+    except Exception:
+      logging.exception('unable to download actual_image_url %s to file %s' %
+                        (actual_image_url, actual_image_file))
+      raise
 
     # Generate the diff image (absolute diff at each pixel) and
     # max_diff_per_channel.
     diff_image = _generate_image_diff(actual_image, expected_image)
     diff_histogram = diff_image.histogram()
     (diff_width, diff_height) = diff_image.size
     self._weighted_diff_measure = _calculate_weighted_diff_metric(
         diff_histogram, diff_width * diff_height)
     self._max_diff_per_channel = _max_per_band(diff_histogram)
 
@@ skipping 309 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))