Confirmations task needs to roll back transactions on failure

[jeremycline]

The confirmations task doesn't handle failed transactions:

InvalidRequestError("This Session's transaction has been rolled back due to a previous exception during flush. To begin a new transaction with this Session, first issue Session.rollback(). Original exception was: Instance '<QueuedMessage at 0x7fd0dd8fcc90>' has been deleted, or its row is otherwise not present.",)
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/celery/app/trace.py", line 367, in trace_task
    R = retval = fun(*args, **kwargs)
  File "/usr/lib/python2.7/site-packages/celery/app/trace.py", line 622, in __protected_call__
    return self.run(*args, **kwargs)
  File "/usr/lib/python2.7/site-packages/fmn/tasks.py", line 355, in confirmations
    models.Confirmation.delete_expired(session)
  File "/usr/lib/python2.7/site-packages/fmn/lib/models.py", line 886, in delete_expired
    expired = session.query(cls).filter(cls.created_on < too_old).all()
  File "/usr/lib64/python2.7/site-packages/sqlalchemy/orm/query.py", line 2703, in all
    return list(self)
  File "/usr/lib64/python2.7/site-packages/sqlalchemy/orm/query.py", line 2855, in __iter__
    return self._execute_and_instances(context)
  File "/usr/lib64/python2.7/site-packages/sqlalchemy/orm/query.py", line 2876, in _execute_and_instances
    close_with_result=True)
  File "/usr/lib64/python2.7/site-packages/sqlalchemy/orm/query.py", line 2885, in _get_bind_args
    **kw
  File "/usr/lib64/python2.7/site-packages/sqlalchemy/orm/query.py", line 2867, in _connection_from_session
    conn = self.session.connection(**kw)
  File "/usr/lib64/python2.7/site-packages/sqlalchemy/orm/session.py", line 998, in connection
    execution_options=execution_options)
  File "/usr/lib64/python2.7/site-packages/sqlalchemy/orm/session.py", line 1003, in _connection_for_bind
    engine, execution_options)
  File "/usr/lib64/python2.7/site-packages/sqlalchemy/orm/session.py", line 382, in _connection_for_bind
    self._assert_active()
  File "/usr/lib64/python2.7/site-packages/sqlalchemy/orm/session.py", line 276, in _assert_active
    % self._rollback_exception
InvalidRequestError: This Session's transaction has been rolled back due to a previous exception during flush. To begin a new transaction wi
th this Session, first issue Session.rollback(). Original exception was: Instance '<QueuedMessage at 0x7fd0dd8fcc90>' has been deleted, or its row is otherwise not present.