Issues/872 error pages

Closes #872 

Various error pages can be seen at `/status/404/`, `/status/500/` and `/status/down/`

Needs some design love:

- [x] CSS hardcoded into the `error_base.html` template - needs moving to SCSS.
- [x] page class `error` needs to be added to CSS (presumably quite similar to the `home` class)
- [x] font needs changing and drop shadow adding to pick it out from background
- [x] content section at bottom needs suppressing
- [x] down.html needs generating from the template when all done, and notes added regarding how to do that.