I can only guess here: maybe they were added to more clearly document the public
interface of the classes? If so, I don't think that works. Developers who are
not familiar with the convention will just add a new public method to the class
without updating the interface.
This might seem controversial; in many cases the client code gets longer,
because it needs an extra line for an explicit `return nil`. I still prefer
this, because it makes it clearer which calls can return errors.
