code: def to_string(cls, error_code):
          
          if error_code == cls.ZONE_NOT_FOUND_ERROR:
              return 'Zone not found'
          return super(JLinkReadErrors, cls).to_string(error_code)
docs: Returns the string message for the given ``error_code``.
  
          Args:
            cls (JLinkReadErrors): the ``JLinkReadErrors`` class
            error_code (int): error code to convert
  
          Returns:
            An error string corresponding to the error code.
  
          Raises:
            ValueError: if the error code is invalid.