fix import bug

I was getting the error below because ```import dvidclient``` in `ioOperators/__init__.py` is failing silently, but the error message check in `ioOperators/opExportSlot.py` is not enough to catch the ImportError.

```
File ".../lazyflow/lazyflow/operators/ioOperators/opExportSlot.py", line 16, in <module>
    from lazyflow.operators.ioOperators import OpExportDvidVolume
ImportError: cannot import name OpExportDvidVolume
```