code: function formatMessage (message) {
    return '<error' +
      attr(message, 'line') +
      attr(message, 'column') +
      attr(message, 'severity') +
      attr(message, 'message') +
      ('source' in message ? attr(message, 'source') : '') +
      ' />'
  }
docs: Format error line.
  
  @param {object} message Message object to format
  @return {string}