When Host::proxyReady fails to create the SnWatcher proxy (e.g. because
the Watcher has not finished exporting /StatusNotifierWatcher yet), the
cancellable is left set, causing nameAppeared to early-return on every
subsequent event (see the // TODO marker). The Host is then stuck
without a watcher, and the tray module reports
'No such object path /StatusNotifierWatcher' until the bar is fully
restarted.
Clear the cancellable on non-CANCELLED errors and schedule a single
delayed retry of nameAppeared. The guard `watcher_ != nullptr` skips
the retry if a parallel call already succeeded.
Closes#3468
