Field produces both value and error

I'm looking to clarify whether it should be valid for a field to be resolved with both a value and an error in the global errors list.

This scenario came up when resolving a field to an external service. In the event that the external service is down, the server would like to provide a sensible default response whilst still notifying the client that the value is a fallback value.

Its technical possible to achive but I'm wondering whether it is/should be valid as per the spec?

cc @eapache