docs: def require_backup_exists(func):
      """
      Requires that the file referred to by `backup_file` exists in the file
      system before running the decorated function.
      """
      def new_func(*args, **kwargs):
          backup_file = kwargs['backup_file']
          if not os.path.exists(backup_file):
              raise RestoreError("Could not find file '{0}'".format(backup_file))
          return func(*args, **kwargs)
      return new_func