Implement windowsErrorString for WinRT

FORMAT_MESSAGE_ALLOCATE_BUFFER isn't available for WinRT

Change-Id: Ib9cf2a11a4e84f3ff010665bd53b4ffb0d0bfcce
Reviewed-by: Andrew Knight <andrew.knight@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>