Don't retry on 4xx responses (#4601)
* Don't retry on 4xx responses
I'm not sure why this was limited to a small set of 4xx responses.
Nominally, no 4xx request should be retried (in fact the comment
below says this, but then the code didn't quite match it).
This was causing key backup requests to be retried even when the
server responded 404 because the backup in question had been deleted,
meaning the client would retry uselessly and it would take longer for
the client to prompt the user for action.
* Exclude 429s
