Copy files from iOS app bundle to documents directory

/// Asyncronously copies images from exampleImages.bundle to the documents directory and reveals them in the document browser.
    private func prepareExampleImages() {
        DispatchQueue.global(qos: .userInitiated).async {
            guard let documentsDirectory = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first else {
                fatalError("Can't get URL for FileManager.SearchPathDirectory.documentDirectory")
            }
            guard let exampleImageBundlePath = Bundle.main.path(forResource: "exampleImages", ofType: "bundle") else {
                fatalError("Example image bundle missing or unreadable.")
            }
            let fileManager = FileManager.default
            guard let filenames = try? fileManager.contentsOfDirectory(atPath: exampleImageBundlePath), filenames.count == 3 else {
                fatalError("Expected three example images in exampleImages.bundle")
            }
            var destinationURLs = [URL]()
            for filename in filenames {
                let imageURL = URL(fileURLWithPath: "\(exampleImageBundlePath)/\(filename)")
                let destinationURL = documentsDirectory.appendingPathComponent(filename)
                destinationURLs.append(destinationURL)
                do {
                    try fileManager.copyItem(at: imageURL, to: destinationURL)
                } catch {
                    // file exists error: running tutorial with files of this name already present
                    //  that's ok, we don't want to overwrite user data, skip this file
                    //  In some locales, the error.localizedDescription fails to load, so the error code is returned instead.
                    if error.localizedDescription.contains("exist") || error.localizedDescription.contains("ERR516") {
                        // skip this file
                        continue
                    } else {
                        // unknown errors are not handled; skip file
                        // TODO unified error handling show the user a message about example image not loading
                        assertionFailure("Unknown error while copying image from example bundle to documents directory!")
                        continue
                    }
                }
            }
            self.exampleImageURLs = destinationURLs
            // tell document controller to reveal the example images, which updates last used date
            // means if we do it in reverse order they will show up in the correct order in Recents tab (most recently revealed first)
            destinationURLs = destinationURLs.reversed()
            // delay to make sure the calls arrive at the remote view controller in this specific order
            var delay = 0.0
            for url in destinationURLs {
                DispatchQueue.main.asyncAfter(deadline: .now() + delay) {
                    self.presenterVC.documentBrowserVC.revealDocument(at: url, importIfNeeded: true) { (url, error) in
                        // unknown error TODO universal user-facing error handling
                        assert(error == nil, "UIDocumentBrowserViewController can't reveal the image! \(error!.localizedDescription)")
                    }
                }
                delay += 0.2
            }
        }
    }