[ECS][Request]: improve description when Docker Image is not Found

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**Tell us about your request**

This feature request (to provide greater clarity when a Docker image is not available) is a UX improvement that will aid issue diagnosis and give the ECS product a more finished feel.

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

ECS Fargate, possibly ECS.

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**

The issue I'm trying to solve is that the error message provided when a Docker Image is not available is not likely to be understood by an ECS user.

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

Institutional knowledge.

**Additional context**

Obviously, I made a mistake by trying to launch the `latest:latest` image but the error message `"InternalError: failed to create container model: failed to normalize image reference"` is not going to be understood by anyone. If you search for this phrase (`failed to normalize image reference`) on the Internet you'll see a number of posts.

If you wanted to _really_ have a good user experience you could link to a troubleshooting document or state clearly "the image tag ${image_tag} does not exist in the specified repository".

**Attachments**

Start with the issue's ECS Fargate example and the reported “failed to normalize image reference” error; this roadmap repository names no source files or tests. Determine the expected behavior for an unavailable Docker image, and consider the work complete when ECS presents a clear, actionable message or troubleshooting reference to users.

Written by the indexing model from the issue text.