Added clarification to an error message to give better debugging information.
Removed using dao_get_reply_to_by_service_id in tests to be more consistent with other code and use the test db functions or remove the need for a call altogether making the code less complex.
this means that if the task is accidentally ran twice (eg we autoscale
notify-celery-worker-beat to 2), it won't send letters twice.
Additionally, update some function names and config variables to make
it clear that they are referring to letter jobs, rather than all letter
content
