Add error pages to the nginx service
The nginx instance doesn't show pretty error pages. Effectively, all error pages are rendered by the calculator app. Below is an example of the nginx errors:
Following on from #180 (closed), it would be good to be able to present non-specific error pages from nginx, especially in the case of not being able to correctly communicate with the calculator service.
I'm thinking something like GitHub's unicorn error pages (though of course far more original):