Stuck on splash screen when exception occurs and we're resuming to splash screen
## 🐛 Bug Report

I'm not sure if this is two bugs or just one. In a minimal app with splash screen, one activity, and custom app start class, when you navigate with the HW back button, the app closes and when you then start the app, it *resumes* the app from the launch screen and still triggers ApplicationStartup. If an exception occurs in that method this time, the exception is not handled and the app gets stuck on the splash screen.

### Expected behavior
The exception should be caught and the app should terminate or crash, not get stuck.
Maybe also: When we navigate back from "home view" and the app is backgrounded but not stopped, we should resume to the home view.

### Reproduction steps
With a minimal project based on the sample provided in the readme, add a class dervied from MvxAppStart. Add an instance variable ```bool firstStartDone;``` and override ApplicationStartup like this:
```csharp
protected override Task<object?> ApplicationStartup(object? hint = null)
{
    if (firstStartDone)
        throw new ArgumentException();

    firstStartDone = true;
    return base.ApplicationStartup(hint);
}
```

- Start the app, wait for the launch screen to disappear and the home view to show up, then hit the HW back button
- Start the app again
- Optionally, observe with breakpoints that the second start again launches the launch screen. This involves calling methods in MvxSetupSingleton and MvxAppStart, ending up with a call to ApplicationStartup, where ```firstStartDone``` is now still true because the app was not restarted. But the way this Task returning method is being asynchronously called along the way (but rooted in a non-async void method), the exception is not handled -- as opposed to throwing it always, which during normal startup terminates the app.

### Configuration

**Version:** tested with 7.0.1 and 8.0.2

**Platform:** 
- [ ] :iphone: iOS
- [x] :robot: Android
- [ ] :checkered_flag: WPF
- [ ] :earth_americas: UWP
- [ ] :apple: MacOS
- [ ] :tv: tvOS
- [ ] :monkey: Xamarin.Forms
