During upgrade from a previous 15.0 or 15.0.1 version to Adaptive Server 15.0.2 you should have created an restore_msgs.ebf file. Now you must run this script to restore your messages back to the version from which you upgraded.
isql -S -Usa -P -S -i<restore_msgs.ebf>
If you upgraded from a pre-15.0 or 15.0.1 release and
are reverting to a pre-15.x release then you must only run instmsgs.ebf file.
