Raise exceptions in MQTTMessageInfo.wait_for_publish()

MQTTMessageInfo.wait_for_publish() will now raise exceptions if called when
the publish call produced an error.

Closes #550. Thanks to jps-aldridge.

Author: ralight

ChangeLog.txt

@@ -3,7 +3,9 @@ v1.6.0 - 2021-xx-xx
 
 - Changed default TLS version to 1.2 instead of 1.0.
 - Fix incoming MQTT v5 messages with overall property length > 127 bytes being
-incorrectly decoded. Closes #541.
+  incorrectly decoded. Closes #541.
+- MQTTMessageInfo.wait_for_publish() will now raise exceptions if called when
+  the publish call produced an error. Closes #550.
 
 
 v1.5.1 - 2020-09-22