make exception follow pep8

To be critical, Guido said in pep8 is

    When binding caught exceptions to a name, prefer the explicit name binding syntax added in Python 2.6:
    try:
        process_data()
    except Exception as exc:
        raise DataProcessingFailedError(str(exc))
    This is the only syntax supported in Python 3, and avoids the ambiguity problems associated with the older comma-based syntax.