Added the possibility to create settings

I think it's a good idea to have the possibility,
to change the title and the name of the CMS via a
configuration file. Therefor I created the class Settings
and a configuration file in config/cms/fortress/global_settings.yml.

Furthermore, I moved the Error class into a Cms::Fortress namespace.
I changed the raise and rescue code to the new Error class

I would also like to insert the config/role.yml to the global_settings.yml
because then we have a proper namespace.