Having a standard new property to place arbitrary data is actually something that has been discussed in the past. It's a family long conversation, but it might interest you: Error "detail"
The main use case being solved there was about making it easier to build nicer error messages using arbitrary data, which is perhaps a little different than what you're going for, but I'd also be happy with simply having an official place to put data for errors. I'm probably a minority in that regard though, because as mentioned before, you can already put arbitrary data anywhere on the error object. The only benefit you gain from having an official property for it is that it removes the slight risk of the committee adding a new property to error objects with the same name you chose, which, while it probably wouldn't break your code if they do that (since your property would override theirs), it would still make the code very confusing.