Catch ConnectionError on Py3.

This is a very boring possible fix for #442.

@joneskoo, do you might testing this branch out to see if it resolves your problem? Actually testing this is stupid-hard, you basically need to rely on a timing window.
So, the issue is that I don't think you can import generic exceptions from all namespaces. Observe:

```python
>>> import requests
>>> requests.IOError
AttributeError: 'module' object has no attribute 'IOError'
```

So we definitely need to add something to the namespace in Py3.