Merge pull request #2706 from ludovic-henry/fix-unhandledexception

[runtime] Set exit code on unhandled exception even if a managed handler is present

In case we would add a AppDomain.CurrentDomain.UnhandledException callback, we would not exit the VM in case of unhandled exception.
This goes against the expected behaviour, as we should indeed call these managed callbacks in all cases, but they should not stop the VM from exiting.

Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=39077