watch method
Lookup a watch by its unique identifier associated with this HSkySparkClient.
If not found return null or raise UnknownWatchException based on checked flag.
Implementation
@override
HWatch? watch(String id, {bool checked = true}) {
HWatch? w = _watches[id];
if (w != null) return w;
if (checked) throw UnknownWatchError(id);
return null;
}