Very simple implementation of 404 page.

This is a super simple implementation of a 404 page that fixes #32. In the future, if we want, we could expand it to work more like #33, which puts the 404 page within the single-page app.
So, what about something like `next({type: 'internal', error: 'No post in body'})` and then the error handler can look for `type === 'internal'` and decide to return a 404 or whatever. I feel like something along those lines a) consolidates translation of error conditions to status codes into one place b) gives us a wide-open, app-specific "namespace" for errors (type: internal), and c) would let us handle error logging in the handler as well, instead of slathering it throughout the app.