Ajax.Responders.register({
  // Show progress indicator and hide 
  // all other notices 
  onLoading: function() {
    document.body.addClassName('loading');
  },
  
  onComplete: function() {
    document.body.removeClassName('loading');
  }    
});

Shopify.onError = function(error) {    
  alert('Error: ' + error.message);
}