Return a json dictionary representing this model.

def _to_dict(self):
        """Return a json dictionary representing this model."""
        _dict = {}
        if hasattr(self, 'code') and self.code is not None:
            _dict['code'] = self.code
        if hasattr(self, 'description') and self.description is not None:
            _dict['description'] = self.description
        if hasattr(self, 'error_id') and self.error_id is not None:
            _dict['error_id'] = self.error_id
        return _dict