code: def _update_error_pointer(error, pointer_or_prefix):
      
      if error.pointer:
          error.pointer = '{}.{}'.format(pointer_or_prefix, error.pointer)
      else:
          error.pointer = '{}'.format(pointer_or_prefix)
      return error
docs: Helper function to update an Error's pointer attribute with a (potentially
      prefixed) dictionary key or list index.