Failing to retrieve a certificate does not result in the charm status showing an error.

[Martin Hilton]

I had an autocert certificate that wasn't quite set up correctly. This resulted in the following error in the autocert charm:

2018-08-07 17:02:37 INFO juju-log Running autocert...
2018-08-07 17:02:38 DEBUG install 2018/08/07 17:02:38 ERROR: Failed to fetch certificate: retrieval failure, server returned status code 404 for URL https://autocert.canonical.com/extended.security.staging.ubuntu.com.crt
2018-08-07 17:02:38 DEBUG install ERROR: Client command failed for service 'haproxy', cert 'extended.security.staging.ubuntu.com': Command '['/usr/sbin/fetchcert', '-autocert_host', 'autocert.canonical.com', '-fqdn', 'extended.security.staging.ubuntu.com', '-cert_path', '/var/lib/haproxy/extended.security.staging.ubuntu.com.pem', '-key_path', '/var/lib/haproxy/extended.security.staging.ubuntu.com.pem', '-chain_path', '/var/lib/haproxy/extended.security.staging.ubuntu.com.pem']' returned non-zero exit status 1
2018-08-07 17:02:38 INFO juju-log Successfully ran autocert
2018-08-07 17:02:38 INFO juju-log Managing 1 certificate for haproxy

It would be good if the charm could detect that fetching the certificate had failed and update the status accordingly.