encodeErrorToCapData = (err, encodeRecur) => {
        const errData = encodeErrorCommon(err, encodeRecur);
        return harden({ [QCLASS]: 'error', ...errData });
      }
Even if an Error is not actually passable, we'd rather send
it anyway because the diagnostic info carried by the error
is more valuable than diagnosing why the error isn't
passable. See comments in isErrorLike.

@param {Error} err
@param {(p: Passable) => Encoding} encodeRecur
@returns {Encoding}