Validate that a string is a valid file name.

def _validate_fname(fname, arg_name):
    """Validate that a string is a valid file name."""
    if fname is not None:
        msg = "Argument `{0}` is not valid".format(arg_name)
        if (not isinstance(fname, str)) or (isinstance(fname, str) and ("\0" in fname)):
            raise RuntimeError(msg)
        try:
            if not os.path.exists(fname):
                os.access(fname, os.W_OK)
        except (TypeError, ValueError):  # pragma: no cover
            raise RuntimeError(msg)