Improve error message for properties of relations in aggregate functions (Where queries)

Given the query

Book.where { author.age > avg(author.age) }

Grails currently spits out the compilation error

Cannot use aggregate function avg on property "null" - no such property on class org.example.Author exists

This is no way indicates that you can't use relations in aggregate functions. It just looks like a bug in Grails. The error message either needs improving or relations' properties should be allowed in aggregate functions.

You can use the project attached to GRAILS-8344 for testing. Simply modify the Where query used in BookController.test().