Use HTTP Trailer for errors that arise after response starts

[dlindhol]

Since a LaTiS server streams data as it is being read, an error could arise after starting a 200 response. We could use a trailer to indicate an error to the user.

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Trailer
https://datatracker.ietf.org/doc/html/rfc2616#section-14.40

Also consider trailer use for other features.

[lindholc]

It looks like there's very little support for HTTP/1.1 trailers in the wild. You can add trailers in http4s, but I didn't see a way to set the value of a trailer based on the stream. It also looks like most browsers don't fully support trailers, so many of our clients wouldn't be able to see them even if we could add them.