[BUGFIX] Send header Error 404 in case of page not found errors

If a page is not found, status 404 has to be sent.

Due to a recent change, an Exception is thrown instead of exiting.
As the Exception Handler overwrites the status 404 with a 503, this
code was sent to the client.

Change-Id: Ia6c7686c720863aa80e390c452527cabb338bde3
Resolves: #24945
Reviewed-on: http://review.typo3.org/1245
Reviewed-by: Susanne Moog
Tested-by: Susanne Moog