I'm trying to fill Field with user information using ajax. It work well;

ACTION

  public Operatore User(int Id)
        {
            Operatore User = new Operatore();

            /*
             DO STUFF TO FILL USER

             */

            /*if something go wrong 
              throw new Exception("user does not exist");
             */


            return User;
        }

but if i throw Exception with custom message i can't read it in Ajax

AJAX

     $.ajax({
                        type: "GET",
                        contentType: "application/json; charset=utf-8",
                        url: "@Url.Action("User")",
                        data: { Id: $("#ID").find(":selected").val()},
                        success: function (msg) {
                        /*FILL THE DATA*/
                        /*It works well*/
                        },
                        error: function (error) {
                            alert(error.Message);
                           /*this does not work*/
                        }
                    });

I just need to read my custom error message. The alert return "undefined" How can i do?