code: function reject( xhr, err, headers ) {
  	const e = new ProgressEvent( 'error' );
  	e.error = e.err = err;
  	e.headers = headers;
  	xhr.dispatchEvent( e );
  }
docs: Emits the "error" event on the `xhr`.
  
  @param {XMLHttpRequest} xhr
  @param {Error} err
  @private