Ignore VDD WriteFile() failures if the child process has terminated

If the child process (e.g. door game) has terminated, don't log errors if/when WriteFile() to the mailslot fails. This would be expected as the mailslot is created/owen-by sbbsexec.dll which would also terminate along with the process, thus closing the mailslot. 

Hopefully resolves the errors reported by DesotoFireflite (VALHALLA).