done = function(err, lastReturn) {
				utils.lastReturn = lastReturn;
				if (err === stopPointer) {
					keep = false;
					callback(err);
				} else if (err) {
					callback(err);
				} else if (keep) {
					callback();
				}
			}
Starts the promise chain.

@param  {?} error Error object, to fire the error callback,
  from an error that happened before.
@param  {!Scraper} scraper Scraper to use in the promise chain.
@protected