Proxy should intercept 404s and nicify them

[PeterJCLaw]

GitHub pages does support custom 404 pages, however that means that we'd need to implement them in each of our pages repos. It would be nicer if the proxy could handle this for us, at least for the GitHub Pages proxies (other backends might want a different behaviour).

proxy_intercept_errors seems useful here, as does this Stack Overflow post which shows how each location block can have its own error_page handling.