add non_exhaustive to a couple more types

[cdisselkoen]

Description of changes

Adds non_exhaustive to a couple more error types related to parsing or validation. Most of our public error types related to parsing or validation already had non_exhaustive.

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):

• updates the CHANGELOG appropriately

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.