Fetching video thumbnails in Swifty Dropbox

Hi,

I use the following code to get thumbnail for images and videos (using Swifty Dropbox). It works for images and for many videos but crashes on some other videos. Shouldn't unsuccessful thumb generation be captured in the error section? Is there a Swift counterpart for Android's getThumbnailBuilder? 

func fetchDropboxThumbnail(client: DropboxClient, file: Files.Metadata, toPath: String) {

let destination : (NSURL, NSHTTPURLResponse) -> NSURL = { temporaryURL, response in

            let thumbUrl = NSURL(fileURLWithPath: toPath)

            return thumbUrl

        }

        client.files.getThumbnail(path: file.pathLower, destination: destination).response  { response, error in

            if let response = response {

                print("Assuming thumbnail saved to toPath")

            }

            else {

                print("\(error)")

                // Assuming thumbnail could not be generated

            }

        }

    }