Prevented 404 errors from sf1 stopping the event propegation in a 404 from sf2

Whenever you decide to stop handling errors in sf1 and move to sf2, you want to use the `kernel.exception` in most cases. This patch will:
 - Prevent sf2 errors from being handled in sf1 if you change the `sfFrontController`
 - Prevent booting sf1 in cases where you throw custom 404 errors in sf2.
 - Will continue with the event propegation if sf1 throws an `sf404ErrorException`
It seems that you're only interested in whether the `$first_controller_is_sf1` is `null`. Shouldn't it be something like `$controller_got_matched`?