message: Update RuntimeError in _Read
diff: @@ -118,7 +118,8 @@ class Converter(object):
  that lacked file_identifier require this.
  Raises:
  - RuntimeError: When flatc cannot be invoked.
  + RuntimeError: 1. When flatc cannot be invoked.
  + 2. When json file does not exists.
  ValueError: When the extension is not json or bin.
  Returns:
  