Pass mesage from servlet to ajax in case of error

hi,

I am using ajax to insert a record in database through Servlet.
Record get inserted properly.
the problem is how to send the exception messages in the ajax method so that appropriate  can be displayed to user in case exception occurs?
             
$.ajax({
                         type: 'post',
                         url: 'registdata',
                         data: $('#registdataform').serialize(),
                         success: function (data) {
                              // $("#form")[0].reset();
                               alert("sucess");
                         },
                         error: function(data) {
                             // check status && error

                             
                          },
                             
                         });

how to pass message through servlet in  ajax?