repo_management/convert.py:
Change the docuemntation of `_desc_data_line_to_dicts()` to reflect,
that it may raise ValueError under certain conditions.
Change `_desc_data_to_model()` to try/except ValueError raised by
`_desc_data_line_to_dicts()` and raise a
`errors.REpomanagementValidationError` instead.

tests/test_convert.py:
Change tests expecting to raise `ValueError` to expect
`errors.RepoManagementValidationError` instead.