docs: def system_error(self, msg_id=None, message=None, description=None,
                       validation_timeout=False, exc_info=None, **kw):
          """Add an error message for an unexpected exception in validator
          code, and move it to the front of the error message list. If
          `exc_info` is supplied, the error will be logged.
  
          If the error is a validation timeout, it is re-raised unless
          `msg_id` is "validation_timeout"."""
  
          if exc_info:
              if (isinstance(exc_info[1], validator.ValidationTimeout) and
                      msg_id != 'validation_timeout'):
                  # These should always propagate to the top-level exception
                  # handler, and be reported only once.
                  raise exc_info[1]
  
              log.error('Unexpected error during validation: %s: %s'
                        % (exc_info[0].__name__, exc_info[1]),
                        exc_info=exc_info)
  
          full_id = ('validator', 'unexpected_exception')
          if msg_id:
              full_id += (msg_id,)
  
          self.error(full_id,
                     message or 'An unexpected error has occurred.',
                     description or
                     ('Validation was unable to complete successfully due '
                      'to an unexpected error.',
                      'The error has been logged, but please consider '
                      'filing an issue report here: '
                      'https://bit.ly/1POrYYU'),
                     tier=1, **kw)
  
          # Move the error message to the beginning of the list.
          self.errors.insert(0, self.errors.pop())