If a resource has failed to load, calling loadWithCompletion: on it subsequently will not change its state. The callback will be invoked immediately with the past load error. In order to retry loading the resource, you must useretryLoadWithCompletion: instead. The advantage of this approach is that if, hypothetically, loadWithCompletion:were to retry loading a failed resource, an app could easily end up making many asynchronous requests to load the same resource just because it shared the same resource instance in different parts of the application and each part tried to load the resource without first checking if it was already loading. This pattern allows loadWithCompletion: to be invoked indiscriminately without worrying that it might add overhead, and makes retrying a separate and explicit scenario using retryLoadWithCompletion:.
