Problem

No error log is output when sending Android push notifications with invalid tokens.

Cause

When the FCM API receives an invalid token, it writes an error statement in the response body and returns it with status 200.
However, Gaurun does not support that specification.

Reference

https://firebase.google.com/docs/cloud-messaging/http-server-ref#error-codes

Suggestion

It used to support the specification, but it was deleted by the following commit.
I think it can be solved by referring to this commit and and restoring the deleted implementation.

1704b25