Issues/872 error pages

:warning: NOT READY FOR MERGE YET.

Closes #872 

Various error pages can be seen at `/status/404/`, `/status/500/` and `/status/down/`

Needs some design love:

- [ ] CSS hardcoded into the `error_base.html` template - needs moving to SCSS.
- [ ] page class `error` needs to be added to CSS (presumably quite similar to the `home` class)
- [ ] font needs changing and drop shadow adding to pick it out from background
- [ ] content section at bottom needs suppressing