On RHEL Image servers we return 404 on error of certification.

We do this to prevent leakage of information, we don't want people
to be able to probe for existing content.  When docker is running
through its certificates, it should continue trying with a new
certificate even if it gets back a 404 error code.

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)