Add try-catch to AdsSymbol destructor

[RobertoRoos]

(I'll put this issue here as the pyads-symbols package has not yet started.)

If an ADS connection is closed unexpectedly, the AdsSymbol destructor will throw errors when trying to close ads notifications. This could prevent other application destructors from being run.

I think it's best to quietly continue when the notification re-register fails here, since without an active connection nothing can be done in the first place.