How can I get MissingRegistration or InvalidRegistration errors to show?

[pineappeal]

I have this up and running just fine and there are no issues as far as basic functionality goes.
What I don't seem to be able to do is getting error response messages to show like "200 + error:MissingRegistration" or "200 + error:InvalidRegistration" or "200 + error:NotRegistered"
I am able to get the basic HTTP response code and the response reason (text) but not the detailed FCM responses. This even happens when I send to an incorrect registration id ... the answer is always OK 200. I'm not sure if this is an issue or if I am not calling the correct response method.

[ThibaultNocchi]

Ok that may be a little bit late, but I found the solution by looking at another PHP lib for FCM. When I tried it I got the FCM error name, so I decided to dive into the code (as I don't really know how Guzzle works). You can look at line 75 here and see you can call the methods getBody()->getContents() on your response and you get a JSON string to parse. Have a nice day!