No sensible diagnostics for expired certificates

[Jens Finkhäuser]

I had a running OpenVPN setup that one day "suddenly" failed to connect. It took a while to figure out that the certificates I've been using had expired.

Expected:
- An error message indicating that certificates expired.

Actual:
- Generic failure to connect error message.
- Generic OpenVPN log file contents.

It might be nice to check certificate validity before handing certificates off to OpenVPN to use. If the certificate is expired, the far more user friendly thing to do would be to show a specific error bubble stating that's the case.

$ lsb_release -rd
Description: Ubuntu 16.04.3 LTS
Release: 16.04

$ apt-cache policy network-manager-openvpn
network-manager-openvpn:
  Installed: 1.1.93-1ubuntu1.1
  Candidate: 1.1.93-1ubuntu1.1
  Version table:
 *** 1.1.93-1ubuntu1.1 500
        500 http://de.archive.ubuntu.com/ubuntu xenial-updates/universe amd64 Packages
        100 /var/lib/dpkg/status
     1.1.93-1ubuntu1 500
        500 http://de.archive.ubuntu.com/ubuntu xenial/universe amd64 Packages