Fix docs describing the arguments of formatFailure and formatError

As far as I can tell, based on reading the code of PluginProxy.chain(), the note about `return (test, err)` is simply wrong. Also, some plugin implementors, for example https://github.com/abeld/freshen/blob/master/freshen/noseplugin.py#L230 interpret the documentation as "I don't have to do anything, if I don't modify the error tuple", which I assume is also wrong, since the next formatError or formatFailure method in the chain will get None instead of the error tuple.