def _parsed_error(fs, path, parse_func):
    """If log lines at *path* (as downloaded by *fs*, which in 0.3.5 is a
    runner object but in 0.4+ will be a filesystem object) are matched by
    *parse_func*, return the relevant lines. Otherwise, return None.
    """
    lines = fs.cat(path)
    if not lines:
        return None
    return parse_func(lines)