validate_explain should have a way of getting explanations from non-parameterized type constraints

[tobyink]

The reason why it doesn't is that these are usually very simple type constraints that don't warrant a detailed explanation of why a value didn't pass the constraint.

However in the case of something like JUniqueItems (from Types::JSONSchema), an explanation would be useful.

This could be part of a wider effort to stabilize the API around explanations.