Comment 3 for bug 1815576

I think there is a bug in the python uploader, where a blob would be missing:

Feb 12 06:32:06 standalone.localdomain registry[20249]: time="2019-02-12T06:32:06Z" level=error msg="response completed with error" err.code="blob unknown" err.detail=sha256:e7d3075c4205ccccbd4489430acc058e2958469a08829ae3906e65fd4da1551f err.message="blob unknown to registry" go.version=go1.9.4 http.request.host="192.168.24.1:8787" http.request.id=8cfa411d-904b-4c94-9025-9f6dbade0a64 http.request.method=HEAD http.request.remoteaddr="192.168.24.1:34328" http.request.uri="/v2/tripleomaster/centos-binary-aodh-notifier/blobs/sha256:e7d3075c4205ccccbd4489430acc058e2958469a08829ae3906e65fd4da1551f" http.request.useragent="python-requests/2.19.1" http.response.contenttype="application/json; charset=utf-8" http.response.duration=3.428097ms http.response.status=404 http.response.written=157 instance.id=4071e315-a1b9-4d87-9b9b-11fc81da918b vars.digest="sha256:e7d3075c4205ccccbd4489430acc058e2958469a08829ae3906e65fd4da1551f" vars.name="tripleomaster/centos-binary-aodh-notifier" version="v2.6.2+unknown"
Feb 12 06:32:06 standalone.localdomain registry[20249]: 192.168.24.1 - - [12/Feb/2019:06:32:06 +0000] "HEAD /v2/tripleomaster/centos-binary-aodh-notifier/blobs/sha256:e7d3075c4205ccccbd4489430acc058e2958469a08829ae3906e65fd4da1551f HTTP/1.1" 404 157 "" "python-requests/2.19.1"

It returns 404 which I don't think is considered as a requests.exceptions.RequestException.
If that's the case, the _cross_repo_mount won't retry:
https://git.openstack.org/cgit/openstack/tripleo-common/tree/tripleo_common/image/image_uploader.py#n579

And move on to the next layer, and at the end report a failure.