add non_exhaustive to more error types

[khieta]

Description of changes

Adds non_exhaustive to some error types that would be reasonable to extend in the future (IMO). What prompted this was that I wanted to backport #890 to 3.3.x, but found that I couldn't due to a new variant in SchemaError. This would have been fine if the error type had been non-exhaustive.

Also includes a quick fix to a comment in api.rs.

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).

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• Updates the "Unreleased" section of the CHANGELOG with a description of my change (required for major/minor version bumps).

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• Does not require updates because my change does not impact the Cedar formal model or DRT infrastructure.
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• I'm not sure how my change impacts cedar-spec. (Post your PR anyways, and we'll discuss in the comments.)