add user through cb API, how to get errors?

Hi everybody,
very simple question:
i found this ( www.allmysocials.com/index.php?option=com_flexicontent&view=items&id=237:create-users-through-api ) function to add new users to cb and it works perfectly for me.
My question is how to get errors in case is impossible to add user (for example username is already taken, etc..)?

Thanks in advance