Custom error pages

[mechanicjay]

Official Gitlab Pages explains that for a custom 404 page on your site, simply add a 404.html into the root of your site:
https://docs.gitlab.com/ee/pages/README.html#custom-error-codes-pages

As it turns out, this is trivial to implement:
Add the following line to /src/helper/template.conf:
error_page 404 = /404.html;

Setup this up and is working well in my environment now.

[mechanicjay]

I'll to get a pull request together for this tomorrow.

[YuMS]

See #20 (comment)

#20 give this a try but it is not perfect, so I merged it to a non-master branch for now.