GH-1634: change log level from ERROR to WARN in ServerEndpointsProvider

leshan-tl-cf-server-coap/src/main/java/org/eclipse/leshan/transport/californium/server/endpoint/CaliforniumServerEndpointsProvider.java

@@ -156,7 +156,7 @@ public void onNotification(Request coapRequest, Response coapResponse) {
                         Observation observation = server.getRegistrationStore().getObservation(regid,
                                 new ObservationIdentifier(coapResponse.getToken().getBytes()));
                         if (observation == null) {
-                            LOG.error("Unexpected error: Unable to find observation with token {} for registration {}",
+                            LOG.warn("Unexpected error: Unable to find observation with token {} for registration {}",
                                     coapResponse.getToken(), regid);
                             return;
                         }