Make 'terminate_on' parameter of RETRY() more intuitive

[mvkorpel]

Using the terminate_on parameter of RETRY() seems a bit unwieldy, or it could be more intuitive. Let's say I would like to stop retrying on code 403 ("Forbidden"). In my opinion, the intuitive way to do this is to specify terminate_on = 403. However, this does not have the desired effect, as even code 200 ("OK") will then cause retries. To do the right thing, one must use terminate_on = c(200, 403), or possibly something like terminate_on = c(100:399, 403) to account for all imaginable non-error codes.

The parameter works exactly as documented, but I think it would be great to have a user-friendly way to (by default?) include non-error codes (< 400) on top of the set given in terminate_on. Alternatively, a puzzled user might appreciate a note in the ?RETRY document about the non-error codes.

I originally wrote this down as a comment to pull request #459.