cannot start a transaction within a transaction[sqlite3]

I am using django-celery worker to download web page. And I have below code to ensure saving.

def ensure_save(obj):
    try:
        obj.save()
    except Exception as e:
        time.sleep(10)
        logger.error(u'############# save %s fail, error:%s.Do rerty#############' % (obj.url, e))
        ensure_save(obj)

task:

@shared_task()
def fetch_article(job_id, url, title=None, cat=None, date=None):

    # logger.info(u'fetch_article:%s' % url)
    a = Article(job_id=job_id, url=url)
    try:
        a.page = fetch(url)
    except Exception as e:
        a.status = 2
        a.error_msg = e

    # maybe some db error cause save fail
    ensure_save(a)

I get such error msg while task running:

############# save http://ift.tt/1M71i4E fail, error:cannot start a transaction within a transaction.Do rerty#############

I can not understand what the error say, ensure_save is just retry django model's save method.