Handle AMS message too large error gracefully

[tofu-rocketry]

Currently you get this:

2021-03-15 15:10:05,560 - ssmsend - ERROR - Unexpected exception in SSM: {'status': u'INVALID_ARGUMENT', 'status_code': 413, 'error': 'While trying the [topic_publish]: Message size is too large'}
2021-03-15 15:10:05,560 - ssmsend - ERROR - Exception type: <class 'argo_ams_library.amsexceptions.AmsServiceException'>

As reported here.