Include more information when json parsing fails

client/isolated_format.py

 # Copyright 2014 The LUCI Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0
 # that can be found in the LICENSE file.
 
 """Understands .isolated files and can do local operations on them."""
 
 import hashlib
 import json
 import logging
 import os
@@ skipping 418 matching lines @@
   """Verifies the .isolated file is valid and loads this object with the json
   data.
 
   Arguments:
   - content: raw serialized content to load.
   - algo: hashlib algorithm class. Used to confirm the algorithm matches the
           algorithm used on the Isolate Server.
   """
   try:
     data = json.loads(content)
-  except ValueError:
-    raise IsolatedError('Failed to parse: %s...' % content[:100])
+  except ValueError as v:
+    raise IsolatedError('Failed to parse (%s): %s...' % (v, content[:100]))
 
   if not isinstance(data, dict):
     raise IsolatedError('Expected dict, got %r' % data)
 
   # Check 'version' first, since it could modify the parsing after.
   value = data.get('version', '1.0')
   if not isinstance(value, basestring):
     raise IsolatedError('Expected string, got %r' % value)
   try:
     version = tuple(map(int, value.split('.')))
@@ skipping 123 matching lines @@
     data['files'] = dict(
         (k.replace(wrong_path_sep, os.path.sep), v)
         for k, v in data['files'].iteritems())
     for v in data['files'].itervalues():
       if 'l' in v:
         v['l'] = v['l'].replace(wrong_path_sep, os.path.sep)
   if 'relative_cwd' in data:
     data['relative_cwd'] = data['relative_cwd'].replace(
         wrong_path_sep, os.path.sep)
   return data