Add tests for _insert_task errors
Add tests for _insert_task errors

Add individual tests covering every exception that should happen within
_insert_tasks, BadTaskStateError, TransientError, TombstonedTaskError
and TaskAlreadyExistsError.

rosshendrickson-wf/furious,rosshendrickson-wf/furious,Workiva/furious,mattsanders-wf/furious,beaulyddon-wf/furious,andreleblanc-wf/furious,andreleblanc-wf/furious,Workiva/furious,beaulyddon-wf/furious,mattsanders-wf/furious