Use the onerror event of the XMLHttpRequest:

function aGet(url, cb) {

    var x = new XMLHttpRequest();

    x.onload = function(e) {

        cb(x.responseText)

    };

    x.onerror= function(e) {

        alert(Error fetching  + url);

    };

    x.open(GET, url, true);

    x.send();

}



var dmp = console.log.bind(console); // Dummy callback to dump to console

aGet(/, dmp) // Ok, uses onload to trigger callback

aGet(http://dgfgdf.com/sdfsdf, dmp); // Fails, uses onerror to trigger alert