Please link in cpp generated lib for %s

Raise code

      # files, to register dynamic proto files and messages.
      # pylint: disable=g-explicit-bool-comparison
      if serialized_pb == b'':
        # Cpp generated code must be linked in if serialized_pb is ''
        try:
          return _message.default_pool.FindFileByName(name)
        except KeyError:
          raise RuntimeError('Please link in cpp generated lib for %s' % (name))
      elif serialized_pb:
        return _message.default_pool.AddSerializedFile(serialized_pb)
      else:
        return super(FileDescriptor, cls).__new__(cls)

  def __init__(self, name, package, options=None,
               serialized_options=None, serialized_pb=None,