Further tweak to CST->EST converison

[john-h-kastner-aws]

Description of changes

Another change to make adding source information to ToASTError nicer. Abstract the pattern node.node.ok_or(ToASTError::MissingNodeData) into one utility function so that when the ToASTError needs a source source location I only have to add it in place instead of everywhere the pattern appears.

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The change in this PR is (choose one, and delete the other options):

• A change "invisible" to users (e.g., documentation, changes to "internal" crates like cedar-policy-core, cedar-validator, etc.)

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