You can check the field "clone-parent" of the 'volume-info' element got from the command "volume-list-info". If the filed "clone-parent" is not set it means the split is complete. BTW, what was the issue in checking the "error" string as you are doing now?
Thanks for the answer. It works fine checking the error string. However, if in the next ONTAP version you guys decide to reword the error message, the script will break. It is more reliable to check for a specific field/flag.