code: function _validateConfig (options) {
      // validate writable options
      if (client || options.master) {
        // when we are a slave, we can't adjust the config, except for
        // changing the master url or becoming a master itself (port configured)
        for (var prop in options) {
          if (prop !== 'master' && prop !== 'slave') {
            throw new Error('Cannot apply configuration option "'  + prop +'", timer is configured as slave.');
          }
        }
      }
    }
docs: Validate configuration, depending on the current mode: slave or normal
  @param {Object} options
  @private