How to handle RuntimeException & Ajax?

[Gin Yeah]

I have a page with AjaxFallbackButton on it.  On RuntimeException, I want to
show some error message in the feedbackpanel on the page.  For testing, I
use this one page and one form and the following code in my WebApplication
to try:

    @Override public RequestCycle newRequestCycle(Request request, Response
response) {
        return new WebRequestCycle(this, (WebRequest) request, (WebResponse)
response) {
            @Override public Page onRuntimeException(Page page,
RuntimeException e) {
                Form f = (Form) page.get("form");
                f.error("An error (see below) has occured, we cannot go
on.");
                f.error(e.getCause() == null ? e.getMessage() : e.getCause
().getMessage());
                return page;
            }
        };
    }

First, is this the correct way to handle RuntimeException in general.  How
about Ajax?  How can I Ajax update my page without sending back a new page?

Second, this only works with js disable.  With js enable, the FeedbackPanel
is empty.  What is wrong?