Empty reply when mimeRender fails

Consider the following custom content type combinator:

```haskell
import Data.Proxy
import Network.HTTP.Media ((//), (/:))
import qualified Network.Wai.Handler.Warp as Warp
import Servant

data Test

instance Accept Test where
contentType _ = "text" // "plain"

instance MimeRender Test a where
mimeRender _ _ = error "not implemented"

main :: IO()
main = Warp.run 8888 $ serve (Proxy @ (Get '[Test] Int)) (return 123)
```

When requesting this server you will get an empty reply:

```
$ curl localhost:8888
curl: (52) Empty reply from server
```

If your server is behind nginx you will likely get a standard 404 page from nginx because of that, for instance I get

```
HTTP/1.1 404 Not Found
Server: nginx/1.14.0
...
```

This is very confusing and I spent a few hours to locate this problem when working with a custom template combinators for servant (inspired by `servant-ede` and `servant-stache` which did not match my needs).

I suggest to fix this by issuing a 500 error instead, but I can't find the place where this has to be fixed right now. I will have a closer look at it later.