Proper 404 handling and general error boundary

[kentcdodds]

You'll find much improved error handling (especially for 404s) in this commit: b67065a

Before, the 404 page looked really bad and was useless. Now it looks really good and is useful.

This new error page should even handle the user's preferences and logged in state. If the part of the app that's busted is the root loader, then you'll at least get an ok-ish error page:

Feel free to style that up more if you like.