Use a tween application instead of a catchall route.
Using a catchall route has some drawbacks. Especially, we have no mean to have
a route that would match only if no other one does AND no view matches either.
Said differently, our default handler cannot be plugged on the route level nor
the view level, because it is has to be activated only if nothing else works in
the pyramid application.
Using a tween application allow to handle requests that raises a HTTPNotFound
error, while having the pyramid error handler still active between our tween
app and the outside world.
Related to #4291173