No Warn Message on Missing Statefulset Labels

[alecrajeev]

There is some improvement to logging that I would recommend implementing.

The statefulset for the thanos-receiver has to have the label key controller.receive.thanos.io/hashring in order for the controller to work correctly. However, if there is no statefulset that has a matching label, is does not log a warn. There should be another logging message that does this.