apps: handling h3::Error::Done
apps: handling h3::Error::Done

When running quiche-server I got the following message pretty often
when the session hit the bandwidth limit.

```
[2020-05-19T06:34:28.655009788Z ERROR quiche_apps] 8e01440c13c8788bacc6b0c12265cc4210bafc05 stream send failed Done
```

These are not actual error and saying the sender buffer is full so
no more send_body() is accepted and the app can retry later safely.

cloudflare/quiche,cloudflare/quiche,cloudflare/quiche