Remove TODOs and add logging

I've had a look and while this is looking good, I wonder about 2 things:

• We may have to add a string to the error message to tell in which context this error occurred, e.g. $this->logger->error('OneSignal failed to POST credentials: ' . $e->getMessage());. Without that, it may be difficult to know where an error appeared.
• Do we also require arguments at that point? It might be important to see the variable values as part of the error message too; otherwise we may not be able to tell, why an error occurred.

The stack trace is most likely not necessary, I guess. From the other information above it should be possible to tell how we came to that location.

Automatically closed - issue fixed for 2 weeks with no activity.