Add a better error message when a Func is missing as_mql()

[timgraham]

I have seen this too many times:

  File "/home/tim/code/django-mongodb/django_mongodb_backend/functions.py", line 105, in func
    operator = MONGO_OPERATORS.get(self.__class__, self.function.lower())
                                                   ^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'lower'