keep the cms usable when having unknown brick types

Hi @donaier and @effkay,

this small PR just rescues brick types that have no corresponding class and will throw therefore a `NameError`. While the backend user might not be able to fix this I changed the behaviour so that now everything will keep going normal (the brick configuration page will still be usable) but the unknown brick type won't be selectable.

Thank you very much,
Alex