Improve precision/helpfulness of "at an earlier point" error message for selfrefok miss

[MichaelChirico]

data.table/src/assign.c

Line 526 in 038ce2b

error(_("It appears that at some earlier point, names of this data.table have been reassigned. Please ensure to use setnames() rather than names<- or colnames<-. Otherwise, please report to data.table issue tracker.")); // # nocov

@HughParsonage flagged that telling user "at an earlier point" "something happened" is not the most helpful error to see:

#6419 (comment)

#6419 adds a similar message in a different place. As pointed out by @aitap, the root cause (IINM in both cases) is R doing a shallow_duplicate(): #6419 (comment)

I believe that means there's no real way for us to fully prevent the user from getting their table into a broken state (which would make sense as a reason why we wanted to start tracking .internal.selfref in the first place long ago).

But armed with a more precise understanding of the underlying issue, we can probably give the user an easier time understanding what's gone wrong & what to do about it.