The idea of "***" begining for error messages where come from?
And very thoughtful solution the *** added as "NOP" command also in IS-BASIC and EXDOS. When re-executing previous (edited) commands then easy to press more enters than needed then the error message also entered as command line. But with this smart solution no problem, the error line don't do anything.
With friends and relatives who also used Enterprise we also use *** in the communication for sign of any problems. For example write SMS because don't got milk in the shop then I write:
*** milk not found