How to make ssl errors more verbose for MQTT client? : I have 2 servers, with a very similar installation (one on Debian 8.7, the other on Debian 8.8).

On the first server, when I try to subscribe to a MQTT topic :

    mosquitto_sub -h localhost -t test -p 8883 --cafile /etc/mosquitto/certs/selfsigned.pem -d

I get this clear message which seems to come from OpenSSL (I already know the reason of the error, it is not the goal of my question) :

    Client mosqsub/9647-CIEYY2T7 sending CONNECT
    OpenSSL Error: error:14090086:SSL routines:SSL3_GET_SERVER_CERTIFICATE:certificate verify failed
    Error: Protocol error

On the other server, for the exact same command, I get only this obscure message without the "OpenSSL" explanation :

    Unable to connect (8).

So how could I make openssl more verbose ?
