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.


List of changes:
 - Moved the `kernel.exception` event listener to above the fallback for readability
 - The catch in the fallback that handles the `sf404ErrorException` now throws `NotFoundHttpException` instead of returning null (fixes a logic exception, controllers should return a response).
 - The `kernel.exception` event listener now has a try catch instead of `null ===` check for the response.
It feels that this comment would be more in place in the `README.md` under a new section "Further migration" (or perhaps FAQ?), between "Installation" and Changelog. There you can explain what you can do if you want Symfony 2 to handle your Symfony 1 404 errors.