Avoid returning partial results if required fields cannot be resolved

[jbreeden-sfx]

This is a trimmed down copy-pasta of a comment from #99:

We're currently returning partial results for federated objects whose required fields cannot be resolved.

If you have this (psuedo-code) schema in service 1:

type Query {
  getParents() [Parent]!
}

type Parent {
  id: ID!
  child: Child!
}

type Child implements Node {
  id: ID!
}

and this in service 2:

type Child implements Node {
  id: ID!
  name: String!
}

And run this query:

{
  getParents {
    child {
      id
      name
    }
  }
}

You may get Child objects with an id but no name, if there was an error in service 2, even though it's a required field.

This seems like it breaks the schema agreement to me. The service is now returning an object without some of the required fields. I think in those failure cases we may need to bubble up the null value to the parent, and so on, until we reach a nullable parent. That would keep to the GQL spec.