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.
Does wordpress not return 404s? This seems mildly edge-casey to me. I could imagine something changing in the api return format causing this app to 404 when the underlying data is actually there, just under a different key or something. This solution has the benefit of simplicity, but almost to the point that I might overlook it as a source of the problem in such a case. 

It might be more robust to call next with a different sort of error and translate that to a 404 in the error handler or something? That would call out to another developer "Hey, we're translating this weird error case into a 404 response" a bit more clearly.