code: function unmarshallWrapper (cb) {
    return function (err, value) {
      if (err) return cb(err);
      var m = unmarshall(value);
      cb(m.error, m.value);
    };
  }
docs: Helper for unmarshalling values
  provided to a callback.
  
  @param {Object} cb
  @return {Function} (err, marshalledValue)