PublishingApiWorker handles deleted models

Instead of raising an exception and retrying the job for the next 25 days, the worker should end gracefully if the model does not exist for whatever reason (usually because they've been deleted - see https://errbit.production.alphagov.co.uk/apps/53020d6c0da11585f10000e7/problems/547729b20da115b5cf00098e)