MINOR: ssl: add the SSL error string when failing to load a certificate

Add the SSL error string when failing to load a certificate in
ssl_sock_load_pem_into_ckch(). It's difficult to know what happen when no
descriptive errror are emitted.

Example:
[ALERT]    (1264006) : config : parsing [ssl_default_server.cfg:51] : 'bind /tmp/ssl.sock' in section 'listen' : unable to load certificate chain from file 'reg-tests/ssl//common.pem': ASN no PEM Header Error
1 file changed