theme_status_messages expects the first message is $messages[0]

When there are multiple messages, the key is not used. But when there is only one, the expected key is 0, which is not always correct (for example #1945758: Work around theme_status_messages() limitation to display checkout form errors outside panes).

    if (count($messages) > 1) {
      $output .= " <ul>\n";
      foreach ($messages as $message) {
        $output .= '  <li>' . $message . "</li>\n";
      }
      $output .= " </ul>\n";
    }
    else {
      $output .= $messages[0];
    }

I'm not sure if it is a bug, or if we want to keep the core simple.

Ref #1939082: Convert theme_status_messages() to Twig.