old_contents: # encoding: utf-8
  
  """
  .. codeauthor:: Tsuyoshi Hombashi <gogogo.vm@gmail.com>
  """
  
  from __future__ import absolute_import
  
  
  class NetworkInterfaceNotFoundError(Exception):
      """
      Exception raised when network interface not found.
      """
  
  
  class ModuleNotFoundError(Exception):
      """
      Exception raised when mandatory kernel module not found.
      """
  
  
  class TcCommandExecutionError(Exception):
      """
      Exception raised when failed to execute a ``tc`` command.
      """
  
  
  class EmptyParameterError(ValueError):
      pass
  
  
  class InvalidParameterError(ValueError):
      pass
  
new_contents: # encoding: utf-8
  
  """
  .. codeauthor:: Tsuyoshi Hombashi <gogogo.vm@gmail.com>
  """
  
  from __future__ import absolute_import
  
  
  class NetworkInterfaceNotFoundError(Exception):
      """
      Exception raised when network interface not found.
      """
  
  
  class ModuleNotFoundError(Exception):
      """
      Exception raised when mandatory kernel module not found.
      """
  
  
  class TcCommandExecutionError(Exception):
      """
      Exception raised when failed to execute a ``tc`` command.
      """
  
  
  class TcAlreadyExist(TcCommandExecutionError):
      pass
  
  
  class EmptyParameterError(ValueError):
      pass
  
  
  class InvalidParameterError(ValueError):
      pass
  
subject: Add an error class