mqtt: Fix misleading debug message

Author: jonnor

src/mqtt.coffee

@@ -100,7 +100,7 @@ class Client extends interfaces.MessagingClient
     try
       msg = JSON.parse message.toString()
     catch e
-      debug 'failed to parse discovery message', e
+      debug "failed to parse incoming message on #{topic} as JSON", e
       msg = message.toString()
     handlers = @subscribers[topic]