defaultErrorFormatters doesn't format 404

[cblp]

defaultErrorFormatters is documented as

Default formatters will just return HTTP 400 status code with error message as response body.

but its implementation is

defaultErrorFormatters = ...
    notFoundErrorFormatter = const err404

err404 = ...
    errBody = ""

So, in case of error 404, the message is not present in the response body.

Workaround

do
    serveWithContext api (errorFormatters :. EmptyContext)
  where
    errorFormatters = defaultErrorFormatters{notFoundErrorFormatter}
    notFoundErrorFormatter _ =
        ServerError
            { errHTTPCode = 404
            , errReasonPhrase = "Not Found"
            , errBody = "404 Not Found"
            , errHeaders = []
            }

Proposed solution

Change defaultErrorFormatters.notFoundErrorFormatter to errBody.