Merge: serialization: serialize the Error class and avoid abstract classes with...

This PR implements the serialization of the `Error` class manually, it is declared in `core` above the `serialization` module.

Also updates `nitserial` so that it does not attempt to instantiate abstract classes with factories.

@privat I have doubts about the use of `is_abstract` and `is_class`, is it appropriate?

Pull-Request: #1821
Reviewed-by: Jean Privat <jean@pryen.org>