public ActionResult Notfound()
        {
            Response.StatusCode = 404;
            Response.StatusCode = 500;
            return View();
        }

<system.webServer>
    <httpErrors errorMode="Custom" existingResponse="Replace">
      <remove statusCode="404" subStatusCode="-1" />
      <error statusCode="404" path="/Home/Notfound" responseMode="ExecuteURL" />
      <remove statusCode="500" subStatusCode="-1" />
      <error statusCode="500" path="/Home/Notfound" responseMode="ExecuteURL" />
    </httpErrors>
  </system.webServer>