/**
  * Convert data attribute to a string
  *
  * @param \core_renderer $output
  * @return string
  */
 protected function data_to_string(\core_renderer $output)
 {
     if ($this->data instanceof \Exception) {
         $info = get_exception_info($this->data);
         return $output->fatal_error($info->message, $info->moreinfourl, $info->link, $info->backtrace, $info->debuginfo);
     } else {
         return json_encode($this->data);
     }
 }