Sys.WebForms.PageRequestManager.getInstance().add_endRequest(onEndRequest);

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

    if (error != null) {
        args.set_errorHandled(true);

        var msg = error.message.replace("Sys.WebForms.PageRequestManagerServerErrorException: ", "");                                
        alert("Unexpected error: " + msg);                
    }
}