This needs to retry the insert on a `TransientError`, then probably just raise on the second one.  So, basically: 

```py
try:
    taskqueue.Queue(name=self.get_queue()).add(task)
except taskqueue.TransientError:
    taskqueue.Queue(name=self.get_queue()).add(task)

except (taskqueue.TaskAlreadyExistsError,
        taskqueue.TombstonedTaskError):
    return
```