Add RequestStatus rather than throw exception [SERINA-3]

That allows us to render the errors rather than just die
Allow null output [SERINA-3]
Allow 404 page to render via twig [SERINA-3]
Add ability to check error via status code [SERINA-3]
Clean up render() method [SERINA-3]
Add rendering of custom error screens [SERINA-3]
Use generic error.html instead of specific [SERINA-3]