Standardize how we raise serialization warnings

[sydney-runkle]

Union serializers do a pretty good job of warning, but we are relatively inconsistent with how we do this across different type serializers in pydantic-core.

Let's add more warnings for model like types, etc.

Warnings might also be more helpful with information regarding where in a nested structure we ran into an issue, like with validation errors.

See #1449 for the motivation here, as well as some additional ideas / commentary.