message: Use common exception for unsupported conversion
  Both ValueError and NotImplementedError was
  raised when a conversion in `convert_entity_to_ast`
  was unsupported.
  Common exception: `NotImplementedError`
diff: @@ -497,7 +497,6 @@ def convert_entity_to_ast(o, program_ctx):
  Raises:
  NotImplementedError: if the object conversion is not yet supported.
  - ValueError: if the entity type is not supported.
  """
  logging.log(1, 'Converting %s', o)
  @@ -514,7 +513,7 @@ def convert_entity_to_ast(o, program_ctx):
  'cannot convert entity "{}": object conversion is not yet'
  ' supported.'.format(o))
  else:
  - raise ValueError(
  + raise NotImplementedError(
  'Entity "%s" has unsupported type "%s". Only functions and classes are '
  'supported for now.' % (o, type(o)))
  