old_contents: class SchematecError(Exception):
      pass
  
  
  class SchemaError(SchematecError):
      pass
  
  
  class ConvertationError(SchematecError):
      pass
  
new_contents: class SchematecError(Exception):
      pass
  
  
  class SchemaError(SchematecError):
      pass
  
  
  class ConvertationError(SchematecError):
      pass
  
  
  class ValidationError(SchematecError):
      pass
  
subject: add validation error