In Servant, the ServerError type has an Exception instance https://hackage.haskell.org/package/servant-server-0.20/docs/Servant-Server.html#t:ServerError
You might speculate that when throwing a ServerError using liftIO . throwIO in a Handler, the ServerError is automatically caught and served as a response, but it ain't so: it's treated as just another exception, and the response code is 500.
btw, why am I throwing a ServerError with HTTP code 200?
I think ServerError is somewhat misnamed: in reality, it can represent any type of response, not only errors.
It's a kind of escape hatch from the typing strictures of Servant: it lets you return any status code and respose body for a request, not just the ones specified in the type-level API.
@DiazCarrete I've never really liked using exception throwing as control flow, but Yesod does something similar for all non-200 and can be used to early-exit with a 200. And, since it's there I have (ab)used it.
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