How can I use the MembershipCreateStatus in my controller below to identify errors?

My controller below creates a new user but I would like to catch any errors from CreateStatus and add the error to my modelstate.

   [HttpPost]
        public ActionResult CreateUser(user UserToCreate)
        {
            if (ModelState.IsValid)
            {
                // TODO: If the UserToCreate object is Valid we'll
                //Eventually want to save it in a database

                Membership.CreateUser(UserToCreate.Username, UserToCreate.Password, UserToCreate.Email);

                return Redirect("/");
            }
            //Invalid - redisplay form with errors
            return View(UserToCreate);
        }