Use existing NSURLSessionTaskDelegate method

[aniastrzezek]

This PR fixes error not being thrown when download finishes with error due to eg. lost network connection.

[Thomas101]

This works a treat! I noticed that the commits come from a fork that's some 70 or so commits behind master, so I patched master locally (065acc4) and it worked perfectly from there.

To test, start downloading file - place in aeroplane mode, the file download hangs indefinitely (no promise rejection or resolution). With the fix the promise rejects shortly after being placed offline

[Thomas101]

Just noticed that if you cancel the download task this throws an error as the promise rejection gets called twice in 2.0.1-rc.2. The following fixes that behaviour

- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error
{
  if (error && error.code != -999) {
    _params.errorCallback(error);
  }
}

Happy to open a new PR for this if it helps, we have a fork over at https://github.com/locly/react-native-fs

[aniastrzezek]

@Thomas101 Thanks for your comments!

Fork is based on the last released version (1.5.1). I'll take a look on task canceling issue!