New exceptions from 0.47 are not exported in `__all__`

[TheElementalOfDestruction]

Not sure if this was intended or not (or whether this would qualify as a bug), but the new exceptions OleFileError and NotOleFileError are not listed in the __all__ attribute of olefile.py. As such, they are not part of the top level module and require a user to fetch them directly from olefile.olefile.