libappindicator should check for the StatusNotifierWatcher interface.

libappindicator should check for the StatusNotifierWatcher interface as it indicates that something is actually listening and throw an error if the interface is not found. As it depends on that interface (and it's kin) for functionality.

Example case:

GNOME Shell (and really EVERY other DE except Plasma or Unity) does not have native libappindicator support therefor if an app were to try to use libappindicator for a tray icon you get no tray icon with no clear reason why.

Silent failure isn't cool guys.