Suppress logging of /ready handler errors

This is achieved by returning Status Unavailable (which is exempt from
logging in the logging middleware) instead of an Internal Server Error.

Fixes #810

pkg/ingester/ingester.go

@@ -493,7 +493,7 @@ func (i *Ingester) ReadinessHandler(w http.ResponseWriter, r *http.Request) {
 	if i.lifecycler.IsReady() {
 		w.WriteHeader(http.StatusNoContent)
 	} else {
-		w.WriteHeader(http.StatusInternalServerError)
+		w.WriteHeader(http.StatusServiceUnavailable)
 	}
 }