Don't count 412 or 416 as errors in stats

The backend HTTP servers emit StatsD metrics of the form
<server>.<method>.timing and <server>.<method>.errors.timing
(e.g. object-server.GET.timing). Whether something counts as an error
or not is based on the response's HTTP status code.

Prior to this commit, "success" was 2xx, 3xx, or 404, while everything
else was considered "error". This adds 412 and 416 to the "success"
set. Like 404, these status codes indicate that we got the request and
processed it without error, but the response was "no". They shouldn't
be in the same category as statuses like 507 that indicate something
stopped the request from being processed.

Change-Id: I5582a51cf6f64aa22c890da01aaaa077f3a54202
Merge "Don't count 412 or 416 as errors in stats"