Improve the 404 page when a site (or page) is not found

[StevenDufresne]

We will not launch with all the current sites. We need to redirect all existing sites to the homepage.

[StevenDufresne]

@dd32 @tellyworth There will be some showcase sites that are removed when we launch the new theme. What's the best way to handle those derelict URLs?

[dd32]

Sites being removed from the showcase is not an unexpected use-case, sites are removed all the time (Either because they no longer exist, or they've had to migrate off WordPress (or, WordPress is no longer visibly powering it, even if used on the backend still)).

Currently the 404 template is not great, but making that a good landing page seems like the best option.
I would expect the 404 page to be something like a heading and page content of "Whoops! We couldn't find that! But checkout these great WordPress sites below: "