Add error pages

Similarly to [register_your_interest db75fbcab8](https://github.com/guidance-guarantee-programme/register_your_interest/commit/db75fbcab8654f879b0e17b077067372641e6e4e) by @andrewgarner
Respond with 404 template when an article is not found.

Note: The exceptions_app mechanism used by [Gaffe] is Rack-based. Rails controllers specs do not go through Rack. Request specs do.

[gaffe]: http://open.mirego.com/gaffe/
Avoid using {...} for multi-line blocks

Rubo :cop:
Use default h1 for error page and update copy
Get error pages to use correct layout
Merge branch 'error-pages'