Add missing code for #550.

src/paho/mqtt/client.py

@@ -345,6 +345,11 @@ class MQTTMessageInfo(object):
         """Block until the message associated with this object is published."""
         if self.rc == MQTT_ERR_QUEUE_SIZE:
             raise ValueError('Message is not queued due to ERR_QUEUE_SIZE')
+        elif self.rc == MQTT_ERR_AGAIN:
+            pass
+        elif self.rc > 0:
+            raise RuntimeError('Message not published: %s' % (error_string(self.rc)))
+
         with self._condition:
             while not self._published:
                 self._condition.wait()