title: NSURLErrorDomain error code -997 "Lost connection to background transfer service"
body: <p>I'm trying to download a lot of files.
  My intent is to be able to continue these operations even if the application is no longer active. In order to obtain this that behavior I'm using a background transfer service. So I created a background NSURLSession (not discretionary) and than I create a request for each file I must download. When all requests have been created a launch these operations one after the other. My problem is that I run into an NSURLErrorDomain error. The error code is -997 and its description is "Lost connection to background transfer service".</p>