Improve parse error message on incorrect template slot

[john-h-kastner-aws]

For a policy permit(principal == ?resource, action, resource) the parse error message is expected a entity uid or template slot, found a ??resource statement. First, it's printing ??resource instead of ?resource (easy fix) and second it's confusing that it expects a templates slot even though it found ?resource. It should clarify that is specifically expects ?principal.

[svanderbleek]

I found where the ParseError is happening but the context is lost on me. I see the ? being inserted but can I simply remove that there without breaking other uses? Then I'm not sure how to clarify as described. Maybe this is a little complicated for a first issue.

[john-h-kastner-aws]

The parser and following CST to AST conversion is a little convoluted, so perhaps not a first issue.

For the ?, I'm confident that's just a matter of deleting the extra ?.

For the rest of the error text improvement, it will come down to tracking down where the value of T::err_str() is coming from. It looks like there's some type level shenanigans happening there.

[svanderbleek]

@john-h-kastner-aws I believe it is here.

[svanderbleek]

By taking var to string here we can see the mismatch, how to communicate that exactly I'm not sure though.

[cdisselkoen]

Resolved by #411