jQuery Ajax error handling, show custom exception messages

Is there some way I can show custom exception messages as
an alert in my jQuery AJAX error
message?

For example, if I want to
throw an exception on the server side via href="http://en.wikipedia.org/wiki/Apache_Struts" rel="noreferrer">Struts
by throw new ApplicationException("User name already exists");,
I want to catch this message ('user name already exists') in the jQuery AJAX error
message.

jQuery("#save").click(function
            () {
 if (jQuery('#form').jVal()) {
 jQuery.ajax({
 type:
            "POST",
 url: "saveuser.do",
 dataType: "html",
 data:
            "userId=" +
            encodeURIComponent(trim(document.forms[0].userId.value)),

 success:
            function (response) {
 jQuery("#usergrid").trigger("reloadGrid");

            clear();
 alert("Details saved successfully!!!");
 },

            error: function (xhr, ajaxOptions, thrownError) {

            alert(xhr.status);
 alert(thrownError);
 }

            });


            }
});

On the
second alert, where I alert the thrown error, I am getting
undefined and the status code is
500.

I am not sure where I am going wrong. What
can I do to fix this problem?