Fix Personalize delete edge cases

[james-jory]

Issue #, if available:

Closes #527

Description of changes:

• Added INACTIVE to list of status values that are safe to delete for recommender resource type. Note: this is unique to recommenders since they can be "stopped".
• Added call to describe dataset group resource type before attempting delete in case it's already being deleted.

Description of testing performed to validate your changes (required if pull request includes CloudFormation or source code changes):

• Tested changes by deleting Retail Demo Store stack with Personalize resources provisioned.

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