Sanity check for `-d` (/local/certs/)

[debuglevel]

bbb-install-2.5.sh -d just accepts if there are no certificates in /local/certs/. nginx later fails with an error message which points to /etc/letsencrypt/live/$HOST/fullchain.pem (instead of /local/certs/) because those are softlinks.

Additionally, there is no documentation on how cert and key should be named. You'd have to look into the script to figure this out.

Therefore, the script should check if /local/certs/fullchain.pem and /local/certs/privkey.pem exist and fail otherwise.