consistently use <div class="error"> for user-visible errors

Some parts of the code used <strong class="error">, some used
plain <div> without the "error" class, and some used <div id="toc">,
which I can't fathom why anyone though could be a good idea, ever.
Changed all of these for consistency.

Change-Id: Id9d8f8e9ed1647d2ddbd4e6b27f22aad0781007a