Do not crash if no notifications daemon is running (fix Debian bug #6822...

Solve Debian bug #682271:
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=682271

That bug is a problem for two kinds of users:
1) those who don't run a notifications daemon (including me some of the times that I happen to restart a gnome-shell session without restarting the system),
2) those who automatically start guake at the start of the session, when the daemon is not ready.

In those cases, the notification is not produced. I don't really think that's a problem, actually - useless notifications appearing when you login are not something particularly nice.