static void errorCallback(Call<Void> call, Exception e) {
    doAnswer(a -> {
      ((Callback<Void>) a.getArgument(0)).onError(e);
      return a;
    }).when(call).enqueue(any(Callback.class));
  }
This shows that regardless of success or error, we block on completion
zipkin/src/test/java/zipkin2/internal/AggregateCallTest.java