Reference variables inside custom error messages

Need the ability to reference variables in custom error messages. It makes the guard code cleaner when you can check for a value, then raise a custom error for that value missing. Im trying to use this for Config rule evaluation annotations, and the Config docs show this is not supported.

> Note: Currently, Guard doesn't support referencing variables inside custom error messages.

https://docs.aws.amazon.com/cfn-guard/latest/ug/variables.html

Start by reviewing the issue description and the linked AWS Config Guard variables documentation to understand the unsupported custom-error-message behavior. Identify the Guard rule-evaluation and annotation entry points before proposing how variable references should be handled. Done means custom error messages can reference variables as described, with validation covering the documented behavior.

Written by the indexing model from the issue text.