[Findit] Record error if swarming task completes with exit_code as 2.

appengine/findit/waterfall/swarming_util.py

 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import base64
 from collections import defaultdict
 import json
 import logging
 import urllib
 import zlib
@@ skipping 195 matching lines @@
   return step_isolated_data
 
 
 def _FetchOutputJsonInfoFromIsolatedServer(isolated_data, http_client):
   """Sends POST request to isolated server and returns response content.
 
   This function is used for fetching
     1. hash code for the output.json file,
     2. the redirect url.
   """
+  if not isolated_data:
+    return None
+
   post_data = {
       'digest': isolated_data['digest'],
       'namespace': {
           'namespace': isolated_data['namespace']
       }
   }
   url = '%s/_ah/api/isolateservice/v1/retrieve' % (
       isolated_data['isolatedserver'])
   content = _SendRequestToServer(url, http_client, post_data)
   return content
@@ skipping 124 matching lines @@
   shard_results = []
   for isolated_data in list_isolated_data:
     output_json = _DownloadTestResults(isolated_data, http_client)
     if not output_json:
       return None
     shard_results.append(output_json)
 
   if len(list_isolated_data) == 1:
     return shard_results[0]
   return _MergeSwarmingTestShards(shard_results)