Log more when loading shard output.json fails

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

Index: scripts/slave/recipe_modules/swarming/resources/collect_gtest_task.py
diff --git a/scripts/slave/recipe_modules/swarming/resources/collect_gtest_task.py b/scripts/slave/recipe_modules/swarming/resources/collect_gtest_task.py
index a31ef6f112849b4ed1e2e6dbe824a692b6098e48..fbaa23f1086aa40048a4061bd39d783b30d301cc 100755
--- a/scripts/slave/recipe_modules/swarming/resources/collect_gtest_task.py
+++ b/scripts/slave/recipe_modules/swarming/resources/collect_gtest_task.py
@@ -112,8 +112,9 @@ def load_shard_json(output_dir, index):
 
     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