code: def write_error(self, status_code, **kwargs):
          
          message = default_message = httplib.responses.get(status_code, '')
          # HTTPError exceptions may have a log_message attribute
          if 'exc_info' in kwargs:
              (_, exc, _) = kwargs['exc_info']
              if hasattr(exc, 'log_message'):
                  message = str(exc.log_message) or default_message
          self.logvalue('halt_reason', message)
          title = "{}: {}".format(status_code, default_message)
          body = "{}: {}".format(status_code, message)
          self.finish("<html><title>" + title + "</title>"
                      "<body>" + body + "</body></html>")
docs: Log halt_reason in service log and output error page