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.
If it does not break the autoloading, I think it would be better to have all error classes defined in one place - so new ones are added easily and one can view all possible errors.

Otherwise, this is really a good idea to have these in one module. I'd like to do the same for the status codes (make them constants, so it is obvious what they do). Thanks for adding this.