title: How can I get failed URL from didFailProvisionalNavigation method
body: <p>I'm trying to display error message for my web view, and I need to know url which is not available, so I implemented delegate method:</p>
  
  <pre><code>- (void)webView:(WKWebView *)webView didFailProvisionalNavigation:(WKNavigation *)navigation withError:(NSError *)error {
      [self.addressBar finishLoadingProgressAnimated:YES];
      NSLog(@"%@", webView.URL);
      [self showErrorPageForURL:error.userInfo[NSErrorFailingURLStringKey]];
  }
  </code></pre>
  
  <p>but NSErrorFailingURLStringKey is deprecated, so how can I get failed URL?
  WKNavigation's interface is empty. webView.URL == nil at that moment.</p>