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