Show meaningful error message when CA cert is not readable

[widhalmt]

Sorry for skipping the template but this issue is quite simple to describe:

I had this with the latest release and the devlopment master

• Secure your Elasticsearch instance and only allow TLS connections
• Give a CA certificate to the Elasticsearch module
• Forget to set permissions for allowing your webserver access to the CA certificate

You only get an ugly stack trace without a hint that the certificate file is unreadable.

Since some people use configuration management for configuring Elasticsearch, it's quite common to use a certificate from somewhere in the filesystem. And that tend's to lead to disaster.

[widhalmt]

ref/NC/704890
ref/me