Replace custom exception structs with PyO3's create_exception! macro to ensure
proper Python inheritance. EncryptionError now inherits from PdftractError,
enabling isinstance(e, PdftractError) to return True for all exception types.
Changes:
- Use create_exception! macro for all 8 exception types
- Update map_error_to_py to set attributes via PyErr::value(py).setattr()
- Register exceptions with py.get_type::<T>() in module init
- Add unit tests for hierarchy and attributes
Closes: pdftract-4ewgr
