Negative http error codes from onHttpError.listen when website is unreachable

Is there a documentation about the negative http error codes from `onHttpError.listen`?
The goal is to lead to a custom error page on timeouts.

I simulated a non responding server by loading `https://www.google.com:81` which will never finish.
In this case, the listener receives http error code -2 on Android, and -1001 on iOS.

Can I rely on those codes to not change?

**Demo code:**
```
Widget build(BuildContext context) {
final plugin = FlutterWebviewPlugin();
plugin.onHttpError.listen((httpError) {
print('error code: ${httpError.code}, url: ${httpError.url}');
if ((Platform.isAndroid && httpError.code == '-2') ||
(Platform.isIOS && httpError.code == '-1001')) {
// navigate to custom error screen
// not sure if error codes are stable/reliable
}
});
plugin.onStateChanged.listen((state) {
print('state changed: type: ${state.type}, url: ${state.url}');
});

return WebviewScaffold(
initialChild: Center(
child: CircularProgressIndicator(),
),
url: 'https://www.google.com:81',
hidden: true,
);
}
```

**Console outputs and behaviour differ depending on the platform:**

Android:
```
state changed: type: WebViewState.startLoad, url: https://www.google.com:81/
error code: -2, url: https://www.google.com:81/
state changed: type: WebViewState.finishLoad, url: https://www.google.com:81/
```

Android finishes with a website telling about the timeout:

iOS:
```
state changed: type: WebViewState.shouldStart, url: https://www.google.com:81/
state changed: type: WebViewState.startLoad, url: https://www.google.com:81/
error code: -1001, url: ?, runtime type: WebViewHttpError
```
iOS never finishes loading and shows the progress indicator forever.

Commencez par le comportement de onHttpError.listen montré dans la démo et comparez les sorties d’Android et d’iOS pour le site web inaccessible simulé. Le travail est terminé lorsque la documentation précise si ces codes négatifs sont stables, comment les différences entre les plateformes doivent être gérées et sur quel comportement les utilisateurs peuvent compter pour les pages de timeout personnalisées.

Rédigé par le modèle d'indexation à partir du texte de l'issue.