Nulls in required fields should cause errors and bubble

[vektah]

Fixes #275

From the spec:

If an error is thrown while resolving a field, it should be treated as though the field returned null, and an error must be added to the "errors" list in the response.

If the result of resolving a field is null (either because the function to resolve the field returned null or because an error occurred), and that field is of a Non-Null type, then a field error is thrown. The error must be added to the "errors" list in the response.

If the field returns null because of an error which has already been added to the "errors" list in the response, the "errors" list must not be further affected. That is, only one error should be added to the errors list per field.

Since Non-Null type fields cannot be null, field errors are propagated to be handled by the parent field. If the parent field may be null then it resolves to null, otherwise if it is a Non-Null type, the field error is further propagated to it’s parent field.

If a List type wraps a Non-Null type, and one of the elements of that list resolves to null, then the entire list must resolve to null. If the List type is also wrapped in a Non-Null, the field error continues to propagate upwards.

If all fields from the root of the request to the source of the field error return Non-Null types, then the "data" entry in the response should be null.

[vektah]

This isn't great, I think there is a bigger refactor that needs to happen to align the types used by codegen and the types in the ast.

[mathewbyrne]

🤔 I guess this can't just be a boolean since you need to recurse to Elem.