Fixed the null return when called as Controller

In a very specific corner-case, a 404 from symfony1 wasn't handled properly. This case can only be triggered if your front-controller in sf1 didn't handle the `sf404ErrorException` which makes it hard to trigger as this is handled by default. Next to that, this only triggered if the `sf1` route matched and called this controller. 

It will no longer return null but wrap the `sf404ErrorException` in a `NotFoundHttpException`. It is easier to debug and not pollute exceptions thrown in sf2.