Sys.Application.add_init(AppInit);

function AppInit(sender) {
    Sys.WebForms.PageRequestManager.getInstance().add_initializeRequest(cancelPostBack);
    Sys.WebForms.PageRequestManager.getInstance().add_endRequest(EndRequest);
}

function cancelPostBack(sender, args) {
    if (Sys.WebForms.PageRequestManager.getInstance().get_isInAsyncPostBack()) {
        args.set_cancel(true);
    }
}   

function EndRequest(sender, args) {
    if (args.get_error() != undefined) {
        var errorMessage = "";
        if (args.get_response().get_statusCode() == "200") {
            errorMessage = args.get_error().message;
        }
        else {
            // the server wasn’t the problem...
            errorMessage = "An unknown error occurred... code: " + args.get_response().get_statusCode();
        }
        // do something with the errorMessage here.
        // now make sure the system knows we handled the error. 
        args.set_errorHandled(true);
    }
}