sd-notify: print a debug message when sd_notify() succeds

sd-notify: print a debug message when sd_notify() succeds

… and also when it fails, in the few cases where this wasn't done already.
When I was debugging https://github.com/systemd/systemd/issues/39048, it
was quite confusing that we print copious information about D-Bus messages
and all other steps we're undertaking, but nothing about the sd-notify
messages which are much more important. Add some debug messages for the
cases where we didn't print anything. (The case where $NOTIFY_SOCKET is
not set in left unchanged. The variable is often left unset and we don't
need to spam logs in that trivial case.)