Passing nil to registerMacro error message

[CoffeeFlux]

When passing nil to the registerMacro message, the error message is fairly unhelpful and requires digging into DepCtrl to identify. Something this common should result in a more useful error.

Per the maintainer's request.

[CoffeeFlux]

Error checking on the whole is fairly primitive and could likely be improved in other places as well - I'll update if I get time.