When any task completes,the NSURLSession object calls the delegate’s
URLSession:task:didCompleteWithError: method with either an error
object,or nil if the task completed successfully. If the task is a
resumable download task,the NSError object’s userInfo dictionary
contains a value for the NSURLSessionDownloadTaskResumeData key. Your
app should use reachability APIs to determine when to retry,and
should then call downloadTaskWithResumeData: or
downloadTaskWithResumeData:completionHandler: to create a new download
task to continue that download. Go to step 3 (creating and resuming
task objects).

https://developer.apple.com/library/ios/documentation/cocoa/Conceptual/URLLoadingSystem/NSURLSessionConcepts/NSURLSessionConcepts.html