Improve error message in invalid template slot

[john-h-kastner-aws]

Description of changes

The parser previously failed to lex invalid templates slots, so principal in ?princpl would fail with invalid token. Fix #451.

The error is now

  × failed to parse policy set
  ╰─▶ expected an entity uid or matching template slot, found ?principl instead of ?principal
   ╭─[<stdin>:1:1]
 1 │ permit(principal in ?principl, action, resource);
   ·                     ─────────
   ╰────

Issue #, if available

Checklist for requesting a review

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

• A bug fix or other functionality change requiring a patch to cedar-policy.

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

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

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

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

Disclaimer

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.