try {
    Sys.WebForms.PageRequestManager.getInstance().add_endRequest(onEndRequest);
}
catch (err) {
}

function onEndRequest(sender, args) {
    var error = args.get_error();
    if (error != null) {

        // Set the error handled flag to avoid a runtime error
        // reaching the user.
        args.set_errorHandled(true);

        // Remove the error name from the message
        var msg = error.message.replace("Sys.WebForms.PageRequestManagerServerErrorException: ", "");
        alert(msg);
    }
}