[Bug]: Image save error output is too low-level.

[jglogan]

I have done the following

• I have searched the existing issues
• If possible, I've reproduced the issue using the 'main' branch of this project

Steps to reproduce

Here's the example from the related #712

% container image pull --platform linux/arm64 alpine:latest
% container image save --output image.tar alpine:latest
Error: notFound: "Content with digest sha256:85f2b723e106c34644cd5851d7e81ee87da98ac54672b29947c052a45d31dc2f"
/Users/john/Documents/sandboxes % container image save --platform linux/arm64 --output image.tar alpine:latest
alpine:latest  

This shows the developer level root cause of the problem. The user-level root cause is "I tried to save platform X but could only find content for platforms (A, B)".

With #712 fixed we could still hit this issue if I were to do:

container image pull --platform linux/arm64 alpine:latest
container image save --platform linux/amd64 --output image.tar alpine:latest

So making the error message more user-friendly is still a must.

Current behavior

image save shows the developer-level cause of the failure.

Expected behavior

image save shows the user-level cause of the failure.

Environment

- OS: 26
- Xcode: 26
- Container: 0.6.0

Relevant log output

N/A

Code of Conduct

• I agree to follow this project's Code of Conduct