Improve error message for template slot mismatch

[svanderbleek]

Description of changes

Improve error message when there is a template slot mismatch such as principal = ?resource instead of principal = ?principal.

Issue #391

Checklist for requesting a review

The change in this PR is:

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

I confirm that this PR:

• 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 Dafny model or DRT infrastructure.

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[cdisselkoen]

Looks great but can we add a unit test for the failing case, that checks that the error message is appopriate?