r21115: notify_internal.c needs to remove the table entry if a process has crashed. So

r21115: notify_internal.c needs to remove the table entry if a process has crashed. So
it needs the specific error message.

Make messages.c return NTSTATUS and specificially NT_STATUS_INVALID_HANDLE if
sending to a non-existent process.

Volker
(This used to be commit 3f620d181da0c356c8ffbdb5b380ccab3645a972)