rt.dmain2: Fix Issue 11865 - Unhandled exception does not show crash dialog

If a console window is not present, display uncaught exceptions
in a standard Windows message box.
rt.dmain2: Use core.sys.windows.windows
Merge pull request #703 from CyberShadow/windows-crash-dialog

Fix Issue 11865 - Unhandled exception does not show crash dialog