notificationDaemon: Guard against invalid D-Bus object paths

notificationDaemon: Guard against invalid D-Bus object paths

When making any D-Bus call through the GDBus' proxy wrapper with an
invalid D-Bus object path, gnome-shell hangs.

Supposedly FdoApplicationProxy constructor should validate the passed
D-Bus object path and throw an error if the path is invalid. Since it
does not do that, we work it around by making sure that the deduced
D-Bus object path is valid or throw an exception if the path is not.

https://bugzilla.gnome.org/show_bug.cgi?id=787902