Before, if a site's certificate was not found, the site was served
over http rather than https. Failing open like this is problematic
for sites where security is important. Presumably the user set
`HTTPS_METHOD` to a non-`noredirect` value (or left it unset) for a
good reason; we should honor it even if it means serving error
messages.
WARNING: This change breaks compatibility. Any vhost where all of the
following are true will fail after this change:
* `HTTPS_METHOD` is either unset or set to a value other than
`nohttps`.
* The vhost does not have its own certificate (`default.crt` doesn't
count).
* Clients expect to be able to access the vhost by using plain http
to nginx-proxy.
To get the previous behavior, set `HTTPS_METHOD` to `nohttps` for the
vhost.
