Dmitry Demidenko

Results 2 issues of Dmitry Demidenko

- TourController.next() shows the next step if current one was removed instead the first one - TourController.prev() shows the previous step if current one was removed instead the error

The logic in task mostly moved to special handler class and the task became a minimal [@shared_task](http://docs.celeryproject.org/en/latest/django/first-steps-with-django.html#using-the-shared-task-decorator) instance. Added proper exception handling.