Improve error pages

[rjackson]

Our 404 page is a bit naff for users of the website:

I'd recommend replacing it with one that:

1. Preserves the site's navigation
2. Provides call-to-actions for
   1. Returning to the homepage
   2. Contacting us (digital infrastructure team? main telegram channel?) if the user thinks there is a problem?

In doing so, possibly tone down the silliness of the current one (looks like we inherited it from BBMS, rather somebody on our side explicitly choosing it)

Possibly relevant for 403 and 500 pages too, although I've haven't had a look at those yet.

Open to thoughts & opinions before I put any time into this :)

[chris18890]

TBF I like it because fo the pop culter reference, similar to the one we have in work -