API docs: "no lsif data" error page sucks

Created by: slimsag

If you land on a page without LSIF data, or a page with LSIF data but no documentation data, you get an error page like this:

https://sourcegraph.com/github.com/gorilla/mux/-/docs

It should either be much nicer or (for now) cause the API docs button to be hidden on the repo page.