style: restore styling for 'error-message' class, used by formencode htmlfill

35d3a85fc650 removed some CSS styling that still is needed for reporting
errors, for example invalid username/password in the login form.

The htmlfill library injects messages with 'error-message' class on errors,
like:
<span class="error-message">Please enter an integer value</span>

There is however nothing that adds the 'success' class.