nbm_update raises error if notebook is created new and there is no cell_types*.pk for it

Error:
```python
FileNotFoundError: [Errno 2] No such file or directory: '/home/jaumeamllo/workspace/mine/test_nbmodular/.nbmodular/cell_types_00_core.pk'
```

Instead: indicate that probably nbm_export has not been run yet and needs to be run before any nbm_update