Current clone bundle fetches from Google storage results HTTP/404
and curl exiting 56.  This is basically WAI, so stop emitting
verbose error output whenever that happens.  Also add a few more
curl exit statuses based on chromite history, and document them.

Change-Id: I3109f8a8a19109ba9bbd62780b40bbcd4fce9b76
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/432197


Commit-Queue: Mike Frysinger <vapier@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Tested-by: Mike Frysinger <vapier@google.com>