throw special exception when push token validation fails

src/Exception/FirebasePhpInvalidTokenException.php

+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\firebase_php\Exception;
+
+/**
+ * Exception if the token is invalid according to Google.
+ *
+ * If you get this exception, the token can be deleted.
+ */
+class FirebasePhpInvalidTokenException extends FirebasePhpException {
+}