I FOUND THE SIMS 3 README!!! (Link included, plus save-game error code summary)

My guess is that Error 15 was designed, in an original unpatched base game, to be a catch-all for an error condition that doesn't meet the descriptions of the 14 that came before it. That's not unusual for programmers to do, but more commonly one would call such an unclassified thing Error 999 or something like that -- in the optimistic hope that they haven't designed something that is so strange that there would be more than 1,000 categories of errors to classify someday.