[WIP] Do not expose routing 404 exception for protected route

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes (did fail locally though...)
| Fixed tickets | #2679
| License       | MIT
| Doc PR        | not related

I'm not sure if this is seen as a new feature or as a bug fix, so I just
submitted it to the master branch.
I think you might be able to use `Response::HTTP_NOT_FOUND` not found here.