Fix a potential crash when formatting errors (django-webpack#99)

Author: jleclanche

webpack_loader/loader.py

@@ -87,6 +87,8 @@ def get_bundle(self, bundle_name):
  assets['file'] = ''
  if 'error' not in assets:
  assets['error'] = 'Unknown Error'
+ if 'message' not in assets:
+ assets['message'] = ''
  error = u"""
  {error} in {file}
  {message}