when certificate is not correctly base64 encoded the charm silently fails instead of giving helpful feedback

When the certificate in ssl_cert config is not correctly base64 encoded the charm silently fails instead of giving helpful feedback.

pem certs are base64 encoded, so it is a bit surprising to base64 the base64 encoded pem.

It would be nice if ssl_cert config option detected a pem cert by reading the ------BEGIN and did not try to base64 decode the config value before writing the cert file.