Split SystemException to handle Windows

Split SystemException to handle Windows

Windows has (at least) two error namespaces, both errno and
GetLastResult(). These overlap, so it is important we keep track of
which one we are dealing with.

To make things extra problematic, the BSD socket API normally uses
errno, but on Windows it has been mapped in to the GetLastResult()
namespace.

Try to keep better control of this by using separate classes for the
namespaces.