Log more when loading shard output.json fails

scripts/slave/recipe_modules/swarming/resources/collect_gtest_task.py

 #!/usr/bin/env python
 # Copyright 2014 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 json
 import optparse
 import os
 import shutil
 import subprocess
@@ skipping 94 matching lines @@
   path = os.path.join(output_dir, str(index), 'output.json')
   try:
     filesize = os.stat(path).st_size
     if filesize > OUTPUT_JSON_SIZE_LIMIT:
       print >> sys.stderr, 'output.json is %d bytes. Max size is %d' % (
            filesize, OUTPUT_JSON_SIZE_LIMIT)
       return None
 
     with open(path) as f:
       return json.load(f)
-  except (IOError, ValueError, OSError):
+  except (IOError, ValueError, OSError) as e:
     print >> sys.stderr, 'Missing or invalid gtest JSON file: %s' % path
+    print >> sys.stderr, '%s: %s' % (type(e).__name__, e)
     return None
 
 
 def merge_list_of_dicts(left, right):
   """Merges dicts left[0] with right[0], left[1] with right[1], etc."""
   output = []
   for i in xrange(max(len(left), len(right))):
     left_dict = left[i] if i < len(left) else {}
     right_dict = right[i] if i < len(right) else {}
     merged_dict = left_dict.copy()
@@ skipping 73 matching lines @@
           'failed to process gtest output JSON', traceback.format_exc())
 
   finally:
     shutil.rmtree(task_output_dir, ignore_errors=True)
 
   return exit_code
 
 
 if __name__ == '__main__':
   sys.exit(main(sys.argv[1:]))