Additional reason for Let's Encrypt error (testing w/o cert)

When I tried Omnibus-GitLab, I kept running into errors such as https://docs.gitlab.com/omnibus/settings/ssl.html#lets-encrypt-fails-on-reconfigure

I wanted to keep my work as simple as possible, so I didn't enable any sort of certificates.

As I searched the noted messages, I found several related bugs that were filed and then closed. I wonder if the root cause of these issues relate to people who tried Omnibus-GitLab in a similar fashion. If so, then perhaps this change will help keep our users happier.

CC'ing @gitlab-com/gl-security/appsec , since this covers the test user case where someone doesn't use a certificate for an example.com domain (test-only, per RFC 2606).