The update-mirror cronjob should check that all files are present

[François Marier]

When the rsync fails to copy all of the required files, it won't exit with an error. It will just copy what it can.

So in order to detect permission problems on the server (https://github.com/libravatar/wiki.libravatar.org/commit/3bd7df285405f0d2625f3ac5f36a76b4d1e36cbb) we should explicitly check in the cronjob that all of the necessary files are present in /var/lib/libravatar/slave/certs/:

- seccdn-chain.pem
- seccdn.crt
- seccdn.pem