Add styles to 400 & 500 pages

Right now they're just a generic Rails "something went wrong" page. We can make these styles match the rest of the site and put a useful link back to the index page.

<img width="777" alt="Screenshot 2019-04-25 10 15 50" src="https://user-images.githubusercontent.com/8680712/56747240-33bced00-6743-11e9-9d3a-572eb8ae4851.png">
