Bugfix TypeError/catchError

[CodeLenny]

TypeError: Future#chainRej expects the function it's given to return a Future.

  Actual: new HttpError("Empty value for parameter 'commit_id': undefined") :: Error

  From calling: function (err) {

      if(err instanceof selector || err.message === selector.message) {

        return cb(err);

      }

      return err;

    }

  With: new HttpError("Empty value for parameter 'commit_id': undefined")

• Fix catchError
• pullRequest.postComment -> issue.postComment (https://github.com/octokit/rest.js/issues/712)