error: function(xhr, status) {                   
    var err = null;
    if (xhr.readyState == 4) {
        var res = xhr.responseText;
 
        if (res && res.substr(0,1) == '{')
            var err = JSON.parseWithDate(res);
        if (!err) {
            if (xhr.status && xhr.status != 200)
                err = new CallbackException(xhr.status + " " + xhr.statusText);
            else
                err = new CallbackException("Callback Error: " + status);
            err.detail = res;
        }
    }
    if (!err)
        err = new CallbackException("Callback Error: " + status);
 
    if (errorHandler)
        errorHandler(err, _I, xhr);
}