Help the user by noting that resource has a deletion policy

[bittrance]

I received the below summary on updating a stack from which I had removed the definition of an S3 bucket which had DeletionPolicy: Retain.

Updating stack quest-swarm
ElasticsearchBackups[AWS::S3::Bucket] Remove
Continue? [yN]

On proceeding, Cloudformation does indeed skip the deletion:

UPDATE_IN_PROGRESS 23:19:43+0000  quest-swarm[Stack] User Initiated
UPDATE_COMPLETE_CLEANUP_IN_PROGRESS 23:19:46+0000  quest-swarm[Stack]
DELETE_SKIPPED 23:19:48+0000  ElasticsearchBackups[Bucket]
UPDATE_COMPLETE 23:19:48+0000  quest-swarm[Stack]

I have had better ergonomy. Since we have access to the previous template, we should be able to check if there was a deletionpolicy on resources that are marked for deletion and notify the user. This is likely to be useful not only for Retain but also for Snapshot which may take some time.