Process: Fix error message for startup failure on Windows

Since commit faff2ab0 (Process: Wait for children to terminate on Ctrl+C,
2015-06-30) we do not report the GetLastError() from CreateProcessW()
failure.  Refactor kwsysProcessCleanup to pass the error code in so
that we can always call GetLastError() immediately after a failure.

Change-Id: I2b7950560c8bde2e29070c87f4927c51dca32e39