With DLO and SLO, we validate that we can read the first segment before
sending data to the client; this helps catch auth errors where the user
has access to read the manifest but not the segments.
Sometimes, though, that validation fails for transient reasons; if the
proxy couldn't get enough responses from primaries to determine whether
the object exists (for example), we should send back a 503 to indicate
to the client that it should retry the request.
Change-Id: Ice5358ff85ee2d5fe60785b73b67dea493044a2c
3d787ddff8