Add failed requests to returned batch instance

[mschmo]

Correct me if I'm wrong, but I believe this description "If some of the calls have failed, returns a new FacebookAdsApiBatch object with those calls" means that if some of the batch requests fail the execute method should return a new FacebookAdsApiBatch with the failed calls.

At the moment the only requests that are retried are ones that returned an empty response body.

[mschmo]

This fix would only retry in cases when there is a failure callback defined for the request index. Perhaps I should update that so that the request is added to the retry list regardless of whether there was a failure callback defined for that index. What are the thoughts of the community? Ok updated, as it just makes way more sense.