Improve 404 behavior for new Diviner

Summary:
Ref T988. When the user clicks a link we haven't explicitly resolved before, we send them to the `/find/` endpoint, but currently just 404 if we can't find the relevant documentation.

Instead, display a more user-friendly error message, since we're probably going to have some of these. Also, make the page title much worse.

Test Plan: Hit a 404 via `/find/`, got a nicer page.

Reviewers: chad, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T988

Differential Revision: https://secure.phabricator.com/D8408