Review error handling for (some) 400 responses from datasette

[rosado]

This ticket is added after issues caused by unexpected table name changes in datasette.

The change caused responses to some of our requests (fetchColumnSummary) to return 400 error. Which in turn ended up displaying errorPages/400 page to the user.

We need to revisit:

• why this particular page was displayed (and not generic 500 error, for example, which seems sensible when 3rd party service makes a breaking change)
• how to change the error handling flow in such cases to display more informative error

[stevenjmesser]

The content on the error page can be designed by Maddie (not sure she’s been added to this project on GitHub yet?)