added logging of MQTT payload errors

common/transport/mqtt/src/main/java/org/thingsboard/server/transport/mqtt/adaptors/JsonMqttAdaptor.java

@@ -207,7 +207,7 @@ public class JsonMqttAdaptor implements MqttTransportAdaptor {
         try {
             return new JsonParser().parse(payload);
         } catch (JsonSyntaxException ex) {
-            log.error(payload);
+            log.error("Payload is in incorrect format: " + payload);
             throw new AdaptorException(ex);
         }
     }