Bug: error for "no value for substitution" is confusing when it refers to a prerequisite

[mikemccracken]

stacker version

main

Describe the bug

If you have a stacker file A.yaml that uses a substitution ${{A}}, and then another file B.yaml, that does not use ${{A}}, but does have A.yaml as a prerequisite, then running stacker build on B.yaml still requires a sub value for ${{A}}.

The build fails with the error error: no value for substitution A but does not say that it came from A.yaml.
This is confusing when you look for the use of ${{A}} in B.yaml and don't find it.

To reproduce

No response

Expected behavior

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Screenshots

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Additional context

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