When there's a cross-site AJAX request an error should be shown inside the Error Console. Google Chrome already does this. The message there looks like this: XMLHttpRequest cannot load https://getfirebug.com/. Origin http://fiddle.jshell.net is not allowed by Access-Control-Allow-Origin. Sebastian
Jonas, do you have time to do this? If not, who does?
There's already early patches for this. Though progress appears to have been stalled. I'm not going to have time to work on this no.