See if we can replace with UnhandledException handler

[jamescrosswell]

In order to reliably catch unhandled exceptions on iOS we listen to Runtime.MarshalManagedException rather than AppDomain.UnhandledException. Whilst this seems to work, it has the disadvantage that the args passed to Runtime.MarshalManagedException don't indicate whether the exception is terminal or not (we just assume it is).

Apparently this should be fixed in .NET 10:

• Issues with unhandled exceptions dotnet/macios#15252 (comment)

We'd need to test this out of course. If it works though, we could do away with the Runtime.MarshalManagedException callback.