servant-client doesn't return envelopes for non-2xx status codes

Full reproduction: https://github.com/bergmark/sce

Using servant-0.13.1 and servant-checked-exceptions-2.0.0.0

```haskell
data BadReq = BadReq deriving Show

deriveJSON defaultOptions ''BadReq

instance ErrStatus BadReq where toErrStatus BadReq = badRequest400

type API = Throws BadReq :> Get '[JSON] Value

api :: Proxy API
api = Proxy

server :: Server API
server = pureErrEnvelope BadReq

main :: IO ()
main = do
tid <- forkIO (run 8080 $ serve api server)
manager' <- newManager defaultManagerSettings
print =<< runClientM (client api) (mkClientEnv manager' (BaseUrl Http "localhost" 8080 ""))
killThread tid
```

I expected this to print the Error envelope but instead I get
```haskell
Left (FailureResponse (Response {responseStatusCode = Status {statusCode = 400, statusMessage = "Bad Request"}, responseHeaders = fromList [("Transfer-Encoding","chunked"),("Date","Sun, 15 Jul 2018 17:53:24 GMT"),("Server","Warp/3.2.22"),("Content-Type","application/json;charset=utf-8")], responseHttpVersion = HTTP/1.1, responseBody = "{\"err\":[]}"}))
```

If i change the ErrStatus instance to return `ok200` I get the envelope as expected:
```haskell
Right (ErrEnvelope (Identity BadReq))
```

Is there something I'm missing?