Custom implementation if the run_docset function for the PubSub module used for Subscriptions

Mostly a copy of https://github.com/absinthe-graphql/absinthe/blob/3d0823bd71c2ebb94357a5588c723e053de8c66a/lib/absinthe/subscription/local.ex#L40 but this lets us decide if we want to send the data to the client or not in certain error cases