The Last Error verb will return an integer containing the last error code. For Apple Scripts this will contain the AppleScript error code that you can look up for more information.
The Last Error code is preserved across restarts of XTension by being saved in the database and remains valid outside of the Interface Error Script so you can log this or write it to the log later to discover what happened earlier assuming no other scripting error has overwritten it in the meantime.
Prior to XTension v 9.4.31 the error values were not saved in the database and would return 0 after a restart until the first error occurred in that session.