function decoderError(fatal, opt_code_point) {
    if (fatal)
      throw TypeError('Decoder error');
    return opt_code_point || 0xFFFD;
  }
@param {boolean} fatal If true, decoding errors raise an exception.
@param {number=} opt_code_point Override the standard fallback code point.
@return {number} The code point to insert on a decoding error.