attachLoadersToContext can only throw error at query execution time, not at call-time

[helfer]

This is because loaders get attached for every query in the root resolve function, and we don't know whether the context is an object until we run a query.

We can fix this if we control the server as well, because then we can ensure that the context is an object, and we could throw an internal server error if the context provided is not an object. That's a good argument for using all our tools in combination.

[helfer]

Closing this for now, we can reopen if it ever becomes a concern again.