FCM response (v1): {
  "error": {
    "code": 400,
    "message": "The registration token is not a valid FCM registration token",
    "status": "INVALID_ARGUMENT",
    "details": [
      {
        "@type": "type.googleapis.com/google.firebase.fcm.v1.FcmError",
        "errorCode": "INVALID_ARGUMENT"
      },
      {
        "@type": "type.googleapis.com/google.rpc.BadRequest",
        "fieldViolations": [
          {
            "field": "message.token",
            "description": "The registration token is not a valid FCM registration token"
          }
        ]
      }
    ]
  }
}

?>