old_contents: class Error(Exception):
      """Base exception class for all ufo2ft errors."""
  
      pass
  
  
  class InvalidFontData(Error):
      """Raised when input font contains invalid data."""
  
      pass
  
  
  class InvalidDesignSpaceData(Error):
      """Raised when input DesignSpace document contains invalid data."""
  
      pass
  
new_contents: class Error(Exception):
      """Base exception class for all ufo2ft errors."""
  
      pass
  
  
  class InvalidFontData(Error):
      """Raised when input font contains invalid data."""
  
      pass
  
  
  class InvalidFeaturesData(Error):
      """Raised when input font contains invalid features data."""
  
      pass
  
  
  class InvalidDesignSpaceData(Error):
      """Raised when input DesignSpace document contains invalid data."""
  
      pass
  
subject: Add InvalidFeaturesData