Throw any deferred exceptions set via :meth:`_add_err`

def _maybe_throw(self):
        """
        Throw any deferred exceptions set via :meth:`_add_err`
        """
        if self._err:
            ex_cls, ex_obj, ex_bt = self._err
            self._err = None
            PyCBC.raise_helper(ex_cls, ex_obj, ex_bt)