code: function proxyStream(err, stats) {
      return through.obj(function(chunk, enc, cb) {
          processStats.call(this, chunk, stats);
  
          if (_.isError(err)) {
              this.emit('error', wrapError(err));
          }
  
          cb(null, chunk);
      });
  }
docs: Re-uses existing `err` and `stats` objects. Can be piped.
  @function
  @alias proxyStream
  @param {Error=} err
  @param {Stats=} stats
  @returns {Stream}