Mensajes cuando se hace una llamada usando Ajax

--- tomado de http://pastie.org/818221 visto en la lista de correo de [Proto-Scripty]

Ajax.Responders.register({
  onCreate: function() {
    new Effect.Appear('ajax_loader', { duration: 0.3, to: 0.5 });
    },
  onComplete: function(request, transport, json) {
    if (0 == Ajax.activeRequestCount) {
      new Effect.Fade('ajax_loader', { duration: 0.3, from: 0.5 });
    }
    if(!request.success()) {
      var errorMapping = $H({
        400: ['Bad Request', 'The request contains bad syntax or cannot be fulfilled.'],
        401: ['Authorization Required', 'You need to authenticate to access this page.'],
        403: ['Forbidden', 'The request was a legal request, but the server is refusing to respond to it.'],
        404: ['Page Not Found', 'The requested resource could not be found.'],
        405: ['Method Not Allowed', 'A request was made of a resource using a request method not supported by that resource; for example, using GET on a form which requires data to be presented via POST, or using PUT on a read-only resource.'],
        406: ['Not Acceptable', 'The action you tried to perform on this resource was considered unacceptable.'],
        415: ['Unsupported Media Type', 'The media type you are requesting is unsupported.'],
        422: ['Unprocessable Entity', 'The request was well-formed but was unable to be followed due to semantic errors.'],
        500: ['Application Error', 'An error occurred in the application code. Report sent.'],
        503: ['Service not available', 'The webserver did not respond to the request.'],
        505: ['HTTP Version Not Supported', 'The requested version is not available on this server.']
      });

      var errorMessage = errorMapping.get(transport.status) || ['Unknown Error', 'An error occurred, but could not be determined correctly.'];

      if (transport.responseJSON && transport.responseJSON.error)
         errorMessage = [transport.responseJSON.error.title, transport.responseJSON.error.message]

      var notifyUser = new GrowlNotifier({
        title: errorMessage[0],
        message: errorMessage[1],
        image: "/images/elements/growl_warning.png",
        type: 'error'
      });
    }
  }
});