Mr Flakey provides easy way to do ad-hoc testing of network failure of your async calls. (1) Add the NuGet package (2) On any async network call that might fail and you want to test, stick a call to the Flakey extension method. For instance "Await DownloadAsync().Flakey()" (3) For ad-hoc testing, put a call to MrFlakey.Start() somewhere e.g. in MainPage.xaml construction. To stop using Mr Flakey, remove this call. This way, every .Flakey() thing you await will give the tester the chance - within the app's UI - to simulate failure of the async operation.
$ dotnet add package AsyncMrFlakeyNo README available.