Code review comment for lp:~mordred/drizzle/plugin-slot-reorg

Changed the method naming and plugin::Registry::add error handling based on Brian's feedback. There are a couple of other things - some indentation issues and actually printing the error message in plugin::Registry::add() that still need to be done, but will need to wait until after plugin-base-class and memory-file-move have been merged in, otherwise it'll cause a giant bag of hurt. (and if all goes well, perhaps those can be merge in today too)