Code review comment for lp:~mir-team/mir/improve-attribute-protocol-and-normalize-getters

Re: client errors - in one of those enthusiastic London days we decided that API constructors would never return NULL, but instead return an error object. Because this makes it very easy to write programs that fail in unexpected ways, we further decided that passing an error object into any API call that isn't explicitly a check for error status would die, so that developers get immediate and useful corefile-based feedback.