Broken pipes in hackage-mirror

When trying to mirror the public Hackage server with hackage-mirror, I frequently get the following error:

```
hackage-mirror: : hClose: resource vanished (Broken pipe)
```

(Also see #131. )This kills hackage-mirror (even though I'm using --keep-going), and I have to restart it. This happens at random intervals: sometimes less than a minute after I start mirroring, sometimes several hours.

If the underlying cause can't be fixed, then hackage-mirror should at least retry the connection if the socket closes.