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.
I should note that I'm thinking the error handler could also accept `next({type: 'http', code: 404})` or something to just make it give a 404.