[ECR] [enhancement]: Push fails when image already exists on repository with tag and is immutable

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**Tell us about your request**
Currently, a push fails when an image already exists with the given tag, but it would nice for this to be handled more gracefully or at least errored out in a way that it can be distinguished from other issues

**Which service(s) is this request for?**
ECR

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
Ability to re-run a deploy of the current build when it failed or re-run deploy tasks in CD so it's possible to rollback to old builds

**Are you currently working around this issue?**
No, just living with it

**Additional context**
Original issue [here](https://github.com/aws/aws-sdk/issues/750)

Start by reviewing the ECR immutable-tag push behavior described in this issue and the linked AWS SDK issue 750. Clarify whether the desired outcome is graceful handling or a distinct error for an existing immutable tag, then define completion around supporting reruns or rollbacks without treating the failure as an indistinguishable push error.

Written by the indexing model from the issue text.