Pages should return 404 if an object cannot be found

Permalinks such as [1] currently always return 200 status, even if the thread cannot be found [2].

If a link cannot be found, the page should return 404.

[1] https://lists.apache.org/thread/5j8l2vf9095dnd30kwcdhzg10rc32p3r
[2] https://lists.apache.org/thread/5j8l2vf9095dnd30kwcdhzgl0rc32p3r

Reproduce the two linked thread URLs and trace how permalink requests handle a missing thread. Confirm that an existing object still responds normally, while each unavailable object returns HTTP 404 instead of 200.

Written by the indexing model from the issue text.