mmsd: Add support for messages that fail to send

I noticed that sometimes you will actually get an error from the MMS server saying that sending failed for some reason. I am adding an API in mmsd-tng to tell Chatty that it failed:

https://gitlab.com/kop316/mmsd/-/merge_requests/109

However, I wanted to check with you before merging either to make sure this looks good. If this API looks good, I will test and let you know.