Returns an unambiguous class or raises an exception.

def get_task_cls(cls, name):
        """
        Returns an unambiguous class or raises an exception.
        """
        task_cls = cls._get_reg().get(name)
        if not task_cls:
            raise TaskClassNotFoundException(cls._missing_task_msg(name))

        if task_cls == cls.AMBIGUOUS_CLASS:
            raise TaskClassAmbigiousException('Task %r is ambiguous' % name)
        return task_cls