Notifications: improve null check before AttachToWidget() (bgo#723889)

GtkNotificationArea class returns null always for its Widget property,
which means that every user of INotificationAreaBox should check if
it's null before dereferencing it, not only check if the object is
null.

This was not happening and was causing an apparently harmless warning
to be printed in the console. The warning didn't have enough info
about the exception so now the .Exception() method is used instead,
which prints the whole stacktrace. (This has been changed in
SoundMenuService as well, for consistency.)

This didn't seem to happen in the stable branch because in there the
class used as an INotificationAreaBox is X11NotificationAreaBox, which
seems to be deprecated in master. Nonetheless, this fix should be
backported too because X11NotificationAreaBox seemed to be only used
in the Linux platform, so by backporting it we may be fixing the
issue in other platforms.

Conflicts:
	src/Extensions/Banshee.NotificationArea/Banshee.NotificationArea/NotificationAreaService.cs