Improved exception message for missing serializer model meta option

The default exception when forgetting or mistyping the `model` meta option in a serializer is `AttributeError: 'NoneType' object has no attribute '_meta'`. That won't make a lot of sense to most users without debugging the traceback.

I added a check which raises an AttributeError with the message `Serializer class 'ClassName' is missing 'model' Meta option`. This should make things clearer.

Possibly an even better alternative would be an AssertionError instead of an AttributeError, which is slightly misleading. Also, should `Meta` in the error message be capitalized or not?

Test is provided and runs on all tox targets.