Add AttributeError to __getattr__ in /core/callback.py ElementsCallbackData

__getattr__ should raise a AttributeError when value is not found, not KeyError, which is raised when __getitem__ fails. New function should look like 

class ElementsCallbackData(dict):
    __slots__ = ()

    def __getattr__(self, value):
        try:
            return self.__getitem__(value)
        except:
            raise AttributeError

