Error more eagerly for proper subclasses of generics that we don't know how to validate

[dmontagu]

Closes #2264. (As it fixes some confusing behavior.)

We should go further and make similar changes for other subclasses of generics which don't have a "standard" way to call __init__.

Users who want to support such classes should implement a __get_pydantic_core_schema__ classmethod on the type in question, or if they don't control the type, on a pydantic-specific annotation used whenever that type occurs as a pydantic field annotation.

An example of how to do this is included in tests.test_edge_cases.test_custom_generic_validators

Selected Reviewer: @samuelcolvin

[dmontagu]

please review (also @adriangb for context, I'm not sure if you did something different/more general than this)