message: Add a clear error message for when a doc does not have a title.
diff: @@ -308,6 +308,10 @@ def build_doc_index(src_dir):
  continue
  title_parser = _GetMarkdownTitle()
  title_parser.process(os.path.join(dirpath, base_name))
  + if title_parser.title is None:
  + msg = ('`{}` has no markdown title (# title)'.format(
  + os.path.join(dirpath, base_name)))
  + raise ValueError(msg)
  key_parts = os.path.join(suffix, base_name[:-3]).split('/')
  if key_parts[-1] == 'index':
  key_parts = key_parts[:-1]
  