docs: protected void checkParamsForAuth(AUTH_REQUIREMENTS required) throws IOException {
      if (!clientSecretsFilename.isEmpty()) {
        File clientSecrets = new File(clientSecretsFilename);
        if (!clientSecrets.exists()) {
          throw new IOException(
              "Client secrets file " + clientSecretsFilename + " does not exist." +
              CLIENT_SECRETS_INSTRUCTIONS);   
        }
      } else {
       if (required == AUTH_REQUIREMENTS.CLIENT_SECRETS_ONLY) {
         throw new IOException(
             "Client secrets file has to be specified (API key can not be used)." 
                 + CLIENT_SECRETS_INSTRUCTIONS); 
       }
       if (apiKey.isEmpty() && required != AUTH_REQUIREMENTS.NONE) {
         throw new IOException(
             "Either an API key or a client secrets file have to be specified." 
                 + CLIENT_SECRETS_INSTRUCTIONS);
       }
      }
    }