Improve error messages for invalid template slots

[cdisselkoen]

Category

Internal refactors/changes

Describe the feature you'd like to request

For the following policy

permit(principal == ?blah, action, resource);

the current error message is just

invalid token

(with the source location pointing at the ?).

This could be better.

Describe alternatives you've considered

N/A

Additional context

No response

Is this something that you'd be interested in working on?

• 👋 I may be able to implement this feature request
• ⚠️ This feature might incur a breaking change