See the examples.confd/dp/error_formatting/error-cbs.c ConfD example and the confd_lib_dp man page section “ERROR FORMATTING CALLBACK” (also available as an appendix in the ConfD UG).
You also have an example under “confd_error_seterr” that changes the “CONFD_ERR_TOO_MANY_ELEMS” error. You can do the same thing for other errors too.
Also note that you can, for example, check the context of the user, e.g. if the context is cli in the "struct confd_user_info”.
The error type you are interested in is CONFD_ERRTYPE_MISC and error code CONFD_ERR_NOEXISTS