Align EntityAttrEvaluationError with cedar#745

[cdisselkoen]

Description of changes

Implement the suggestions in #745 as applied to EntityAttrEvaluationError

This type is used both as the opaque struct in the variant of a larger error (SchemaError) and on its own as an error type (for Entity::new()). The guidelines in #745 are ambiguous whether all errors used as standalone error types need to be enums. I made a judgment call to not make this one a one-variant enum, at least in this case.

Issue #, if available

Checklist for requesting a review

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

• A breaking change requiring a major version bump to cedar-policy (e.g., changes to the signature of an existing API).

I confirm that this PR (choose one, and delete the other options):

• Does not update the CHANGELOG because my change does not significantly impact released code.

I confirm that cedar-spec (choose one, and delete the other options):

• Does not require updates because my change does not impact the Cedar formal model or DRT infrastructure.