get_range_from_index should raise a helpful exception when it gets invalid key values.

A user tried to do this:

.get_range_from_index("task_idx_due_date", [self._model_class.__name__.lower(), None], [self._model_class.__name__.lower(), current_date])

under the impression that passing None would mean an open ended range for the second field in the index, which does not work. We should raise an error in this case, informing the user that None is not a valid value for an index field.