Create a simple, branded 404 and 500 page

Right now if you go to a bogus URI with the browser, you correctly get a 404, but the page is just plain text saying "Not Found.".

Similarly, there is a catchall for 500 errors.

We should create branded 404/500 pages to serve if errors are encountered. Some 404s and 500s will terminate in JSON responses, but the catchalls should return simple, branded HTML telling the user to go somewhere more useful (like index.html).