old_contents: 
  
  class FatalError(Exception):
      '''An error from which we cannot recover.
      '''
      def __init__(self, *args, **kwargs):
          super().__init__(*args, **kwargs)
  
  
  class FatalConfigError(Exception):
      '''Something with the configuration is really bad.
      '''
      def __init__(self, *args, **kwargs):
          super().__init__(*args, **kwargs)
  
new_contents: 
  
  class FatalError(Exception):
      '''An error from which we cannot recover.
      '''
      def __init__(self, *args, **kwargs):
          super().__init__(*args, **kwargs)
  
  
  class FatalConfigError(FatalError):
      '''Something with the configuration is really bad.
      '''
      def __init__(self, *args, **kwargs):
          super().__init__(*args, **kwargs)
  
subject: Fix error hierarchy