opengroup() fails with wrong exception

**Original reporter**: _Cpascual_

The documentation says that opengroup() should raise ValueError if it fails to open the group.
Instead it fails with:

```
<type 'exceptions.TypeError'>: not all arguments converted during string formatting
```

in __getnxclass()

I think this is a bug in `__getnxclass()` and it affects other API functions as well (e.g. openpath)
