jQuery Validate Plugin: Set Multiple Messages for Remote Rule

form.validate({
    rules: {
        username: {
         remote: {
               url: 'check_username.php',
                dataFilter: function(data) {
                    var json = JSON.parse(data);
                    if (json.result === true) {
                     return 'true';
                    }
                   return '"' + json.message + '"';
              }
           }
       }
   }
});