Add feature to force typings for classes that inherit from Error

[chandr-andr]

Hello!
I suggest adding a feature that will force users to write typing. It must check in runtime that all class attributes have a specified type and raise a Runtime-like error if they do not.

This feature must NOT be turned on by default.

class Features(enum.Flag):
    ...
    FORCE_ATTRS_TYPING = enum.auto()

    ...

If we have something like this, it should go down with exception:

class MyAwesomeError(Error):
    ...
    some_attr # without type

[pyctrl]

You can check the idea in this commit — 9014257