How to report error to $.ajax without throwing exception in MVC controller?

This is where HTTP status codes come into play. With Ajax you will be able to handle them accordingly.

[HttpPost]
public ActionResult UpdateUser(UserInformation model){
    if (!UserIsAuthorized())
        return new HttpStatusCodeResult(401, "Custom Error Message 1"); // Unauthorized
    if (!model.IsValid)
        return new HttpStatusCodeResult(400, "Custom Error Message 2"); // Bad Request
    // etc.
}

Here’s a list of the defined status codes.