Add custom error pages

[katjam]

Acceptance

• [ ] As a user of the site, I get a branded error screen with helpful information if I encounter an HTTP error (e.g. 404. 50x)

Notes

• Invalid subdomain

[katjam]

@kimadactyl, you had a note on https://trello.com/c/Nti79a71/56-ux-add-custom-error-pages-404-501-etc about invalid subdomain.

Can you elaborate on that?

[katjam]

This is also important for when subdomains get taken down e.g. https://mossley.placecal.org/

We might want to consider catching a few of the known causes of 500 (such as this one) more gracefully also.