MvxNavigationServiceAppStart: Don't swallow exceptions

[nmilcoff]

✨ What kind of change does this PR introduce? (Bug fix, feature, docs update...)

Bug fix

⤵️ What is the current behavior?

MvxNavigationServiceAppStart does not throw exceptions inside Navigate.

🆕 What is the new behavior (if this is a feature change)?

Exceptions are thrown.

💥 Does this PR introduce a breaking change?

No.

🐛 Recommendations for testing

1. Set Playground.Mac as starter project
2. Introduce an Interface like IMyService
3. Inject IMyService in Playground.Core -> RootViewModel
4. Run the app.

📝 Links to relevant issues/docs

Fixes: #2447

🤔 Checklist before submitting

• All projects build
• Follows style guide lines (code style guide)
• Relevant documentation was updated (docs style guide)
• Nuspec files were updated (when applicable)
• Rebased onto current develop

[nmilcoff]

Sorry about that async void, but if we don't await the Navigate call, the exception is not catched.

Maybe someone has a better idea on how to do this?