BaseExceptions subclasses need all parameters of their __init__ methods to be passed to BaseExceptions.__init__, see BaseException.args for that.

This MR will correct the super().__init__ call.

This will also make the Exceptions pickable, which I need for a cache and for multiprocessing.

The interface does not change with this MR.

But for ForbiddenExtraKeysError I moved the

"Extra fields in constructor for {self.cl.__name__}: {', '.join(self.extra_fields)}"

string to __str__ instead of storing it in the object as attribute. As this is a fixed string for the class (which means it does not need be stored) and will be used when actually "" was passed to the constructor, I think this makes more sense.