Add a couple new exceptions, both to signal configuration issues. One can be...

Add a couple new exceptions, both to signal configuration issues. One can be used to signal an error in a certain configuration, leaving to the user deciding whether it was critical or not, and what to do to deal with it. The other one indicates a configuration exception that we cannot recover from (i.e. missing files), and bootstraps a minimal configuration skeleton that we can use to go on momentarily.