Return a value or raise an exception from a retn tuple.

def result(retn):
    '''
    Return a value or raise an exception from a retn tuple.
    '''
    ok, valu = retn

    if ok:
        return valu

    name, info = valu

    ctor = getattr(s_exc, name, None)
    if ctor is not None:
        raise ctor(**info)

    info['errx'] = name
    raise s_exc.SynErr(**info)