More informative error on version mismatch

I spent a lot of time debugging my arduino, trying to figure out why my
calls were all failing. Turned out to be due to the mqtt cli using v5, while
this library only supports v4. By mqtt cli I mean the [hivemq one](https://hivemq.github.io/mqtt-cli/),
which is the first one that shows up on google searches.

Being new to mqtt this burned me for awhile, so I'm hoping this helps the next
person. Both by having this PR show up if they search, and making the error
more informative.

src/TinyMqtt.cpp

@@ -455,7 +455,7 @@ void MqttClient::processMessage(MqttMessage* mesg)
       }
       if (header[6]!=0x04)
       {
-        debug("unknown level");
+        debug("Unsupported MQTT version (" << (int) header[6] << "), only version=4 supported" << endl);
         break;  // Level 3.1.1
       }