newtype instead of bare string in MimeUnrender / ResultStream?

Currently we have

```haskell
mimeUnrender :: Proxy ctype -> ByteString -> Either String a
```

I think that we should have a wrapper over that `String`. The same error is also in `ResultStream` definition:

```haskell
newtype ResultStream a = ResultStream (forall b. (IO (Maybe (Either String a)) -> IO b) -> IO b)
```

If `newrype` is too much at once, I'll be already happy with a type alias. I have no good candidate for the name though.