Just a note for other people that might be cracking their heads trying to meld MacRuby and NSError ** parameters. The answer was already sitting on my computer, I just never through to look at the MacRuby Core Data XCode templates, but that's beside the point. For those of you searching, you just need to do the following.
error = Pointer.new_with_type('@') unless managed_object_context.save(error) NSLog error.inspect NSApplication.sharedApplication.presentError(error) end
NSLog error.inspect will output
what we want.