Fix UI if chef-server is gone (bnc#865072)

The ticket https://bugzilla.novell.com/show_bug.cgi?id=865072 explains
that crowbar behaves bad if the chef-server is gone and not reachable.

In order to display a better usable error page i have integrated more
exception classes that gets catched by the application controller to
display a proper error page if there are documents not found or the chef
server is gone.
IMHO my approach is better because you don't even have to open any file to see the available exception classes, but i think that's some kind of personal taste.