/_api/web/GetFolderById('{$guid}')?$expand=Folders,Files

{
    "error": {
        "code": "-2147024894, System.IO.FileNotFoundException",
        "message": {
            "lang": "en-EN",
            "value": "File Not Found."
        }
    }
}

{
    "error": {
        "code": "-2147024891, System.UnauthorizedAccessException",
        "message": {
            "lang": "en-EN",
            "value": "Access denied. You do not have permission to perform this action or access this resource."
        }
    }
}