[ECR] [request]: More Specific Exceptions for throttled Image Scans

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**Tell us about your request**
What do you want us to build?
When attempting to scan an image that has already been scanned, the exception produced looks exactly like an API throttling exception instead an "already been scanned" exception.

Having a different exception for images that have already been scanned will be less confusing for customers and make them easier to handle in code.

**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?**
An image can only be scanned once per day – no problem. However, once an image has been scanned, and trying to initiate a new scan from the CLI, it only gives “Throttling Exception”. Because this looks just like an actual Throttling Exception, there is no reasonable way to quickly skipped over an image in this state. Ideally there would be a discrete exception along the lines of “you’ve scanned this already” that I could handle.

**Are you currently working around this issue?**

**Additional context**

**Attachments**
If you think you might have additional information that you'd like to include via an attachment, please do - we'll take a look. (Remember to remove any personally-identifiable information.)

Start by reviewing the ECR image-scan behavior described in the issue and how the CLI surfaces throttling responses. Done means an image scanned within the daily limit produces a distinct, actionable exception rather than the generic throttling exception.

Written by the indexing model from the issue text.