feat(federation): Include originalError in downstreamServiceError

[coryvirok]

 new ApolloServer({
  formatError: GraphQLError => GraphQLError.originalError, // undefined
  gateway: new ApolloGateway({ ... })
  ...
})

The GraphQLError type expected to have originalError (if avialible)
formatError: GraphQLError => GraphQLError.originalError

In the event of a downstreamServiceError, it does not include the original error.
I believe it's reasonable to expect the original error in this event.

---

Originally at: apollographql/apollo-server#3211

[apollo-cla]

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[coryvirok]

Originally apollographql/apollo-server#3211 but it looks like the author hasn't created the PR in this repo yet and this is a pretty important feature to help debug downstream errors.

@bl42 I'm happy to close this if you want to open a new PR and retain authorship. Otherwise, I'd love to see this change make its way into main.