Since FormClientCallback does not put mgnlLoginError request attribute into Freemarker data model one cannot "capture" such attribute and display error message to the user when login fails on Form based authentication.

Further investigation shows that this would not be possible since base interface does not expose request object (only response is available) so options are:

1. use 401 error page (this did not work even as I declared a simple html 401 page)
2. expose request in base interface and abstract class and then in FormClientCallback grab the mgnlLoginError if set and put it into freemarker data set (hash map)
3. expose request object in every freemarker call (just like contextPath is exposed, if request is exposed contextPath can be ommited actually...) (see FreemarkerHelper.render)