Make _merge_isolated_script_shards robust to harness failures.

scripts/slave/recipe_modules/swarming/api.py

 # 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 datetime
 import functools
 
 from recipe_engine.types import freeze
 from recipe_engine import recipe_api
 
@@ skipping 725 matching lines @@
     merged_results = {
       'successes': [],
       'failures': [],
       'valid': True,
     }
     for i in xrange(task.shards):
       path = self.m.path.join(str(i), 'output.json')
       if path not in step_result.raw_io.output_dir:
         raise Exception('no results from shard #%d' % i)
       results_raw = step_result.raw_io.output_dir[path]
-      results_json = self.m.json.loads(results_raw)
+      try:
+        results_json = self.m.json.loads(results_raw)
+      except Exception as e:
+        raise Exception('error decoding JSON results from shard #%d' % i)
       for key in merged_results:
         if key in results_json:
           if isinstance(merged_results[key], list):
             merged_results[key].extend(results_json[key])
           elif isinstance(merged_results[key], bool):
             merged_results[key] = merged_results[key] and results_json[key]
           else:
             raise recipe_api.InfraFailure(
               'Unknown key type ' + type(merged_results[key]) +
               ' when handling key ' + key + '.')  # pragma: no cover
@@ skipping 238 matching lines @@
 
   def get_shard_view_url(self, index):
     """Returns URL of HTML page with shard details or None if not available.
 
     Works only after the task has been successfully triggered.
     """
     if self._trigger_output and self._trigger_output.get('tasks'):
       for shard_dict in self._trigger_output['tasks'].itervalues():
         if shard_dict['shard_index'] == index:
           return shard_dict['view_url']