improve server-side error logging

### Describe the problem

Realised I wrote this in #13848 but didn't make a note of it anywhere else:

> We could probably improve logging of server-side errors all round (e.g. a nicely formatted `[500] GET /whatever` with a sanitized stack trace, maybe a code frame if appropriate, maybe opening a modal on the client, etc) 

### Describe the proposed solution

All of the above