[4.0] Change emitted type of client.subscribe(...) #12456
Description
Subscriptions currently emit a FetchResult type which is meant to be representative of a raw GraphQL response. #12427 is doing work to unify error behavior between network errors and GraphQL errors which means that the errorPolicy will change the behavior for both types. Currently network errors always trigger errors in subscriptions, regardless of the errorPolicy. Since FetchResult doesn't allow for an error field in its type, we can't convert the existing behavior over to match queries.
By moving to an ApolloQueryResult, we can unify both the behavior of subscriptions with queries and ensure that errors are accessed in the same way.