servant-server returns errors that don't go through a formatter

[neilmayhew]

405 "Method Not Allowed" isn't formatted:

$ curl -v http://localhost:8080/post-endpoint
*   Trying ::1:8080...
* Connected to localhost (::1) port 8080 (#0)
> GET /post-endpoint HTTP/1.1
> Host: localhost:8080
> User-Agent: curl/7.70.0
> Accept: */*
> 
* Mark bundle as not supporting multiuse
< HTTP/1.1 405 Method Not Allowed
< Transfer-Encoding: chunked
< Date: Sat, 26 Sep 2020 16:33:56 GMT
< Server: Warp/3.3.12
< 
* Connection #0 to host localhost left intact

404 "Not Found" is formatted:

$ curl -v http://localhost:8080/nonexistent-endpoint
*   Trying ::1:8080...
* Connected to localhost (::1) port 8080 (#0)
> GET /nonexistent-endpoint HTTP/1.1
> Host: localhost:8080
> User-Agent: curl/7.70.0
> Accept: */*
> 
* Mark bundle as not supporting multiuse
< HTTP/1.1 404 Not Found
< Transfer-Encoding: chunked
< Date: Sat, 26 Sep 2020 17:01:22 GMT
< Server: Warp/3.3.12
< Content-Type: application/json
< 
* Connection #0 to host localhost left intact
{"msg":"Not found path: /other-endpoint"}

You can see the various places where unformatted errors are returned in the code:

$ git grep -E 'Fail.*err[0-9]+' servant-server/src
servant-server/src/Servant/Server.hs:  routerLayout (route p context (emptyDelayed (FailFatal err501)))
servant-server/src/Servant/Server/Internal.hs:  | otherwise                    = delayedFail err405
servant-server/src/Servant/Server/Internal.hs:  | otherwise                   = delayedFail err406
servant-server/src/Servant/Server/Internal.hs:                 Nothing -> FailFatal err406 -- this should not happen (checked before), so we make it fatal if it does
servant-server/src/Servant/Server/Internal.hs:              accCheck = when (isNothing cmediatype) $ delayedFail err406
servant-server/src/Servant/Server/Internal.hs:          Nothing -> delayedFail err415
servant-server/src/Servant/Server/Internal/BasicAuth.hs:       Unauthorized   -> delayedFailFatal err403
servant-server/src/Servant/Server/Internal/BasicAuth.hs:  where plzAuthenticate = delayedFailFatal err401 { errHeaders = [mkBAChallengerHdr realm] }

[maksbotan]

Hmm, as far as I understand HTTP spec, 404 and 405 responses are not expected to have a body anyway. Do you disagree?