I have added a custom button to the server ribbon in SharePoint (I have used a feature with Farm scope, so that the button is visible throughout the various site collections).

For the elements of the feature, I have added a CustomUIExtension through which I want to load an aspx page on the click of the button.

<CommandUIHandler
    Command="Test_Button"
    CommandAction="javascript:
      function demoCallback(dialogResult, returnValue)
      {
        SP.UI.Notify.addNotification('Operation Successful!');
        SP.UI.ModalDialog.RefreshPage(SP.UI.DialogResult.OK);
      }

      var options = {              
              url: '/_layouts/CustomPage.aspx',
              tite: 'Custom Page',
              dialogReturnValueCallback: demoCallback };
      SP.UI.ModalDialog.showModalDialog(options);" 
/>

I have added the CustomPage.aspx and its corresponding code behind class to the 14 hive (inside 14/TEMPLATE/LAYOUTS). However when I install the feature and click the button, I get an error saying "Cannot load CustomPage".

I understand that I haven't deployed the assembly, but shouldn't the aspx page be compiled Just In Time?