Support task retries

[OmarWKH]

Retrying tasks causes an error.

I think it's because Progress.get_info reaches the last line and returns an exception. The view tries to send it as JSON but can't.

celery-progress/celery_progress/backend.py

Line 95 in af964fe

return self.result.info

Example task:

def retry_task():
    try:
        raise StopIteration('We broke it!')
    except StopIteration as exc:
        raise self.retry(exc=exc)

[EJH2]

Fixed in #61