Add error pages

- Uses [Gaffe] to handle exceptions
- Follows [content design from GOV.UK](https://github.com/alphagov/static/tree/master/app/views/root)

This will protect us if the application can start, but we may need to generate a completely static page that can be served upstream?

[gaffe]: http://open.mirego.com/gaffe/
Using `content_for?` instead of `content_for` might be clearer?