Stop polling notes when response is not 200

Follow up of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15140

We can prevent some useless requests if we stop polling notes when response is not 200.
Notes polling continue even if user loses access to a noteable or a legit error is happening.

Suggestion:

Fix notes.js adding clearInterval(Notes.interval); to the error callback:

  getContent() {
    if (this.refreshing) {
      return;
    }
    this.refreshing = true;
    return $.ajax({
      url: this.notes_url,
      headers: { 'X-Last-Fetched-At': this.last_fetched_at },
      dataType: 'json',
      success: (function(_this) {
        return function(data) {
          var notes;
          notes = data.notes;
          _this.last_fetched_at = data.last_fetched_at;
          _this.setPollingInterval(data.notes.length);
          return $.each(notes, function(i, note) {
            _this.renderNote(note);
          });
        };
      })(this)
    }).always((function(_this) {
      return function() {
        return _this.refreshing = false;
      };
    })(this));
  }