fix: return underlying sasl error message

[dnwe]

The SASL Authentication response message from Kafka has an additional field which can contain a description of why the authentication failed. Currently we drop this in Sarama and just return a generic message based on the kError code.

[dnwe]

@k-wall thoughts?

aside: I wonder if we need to tweak/condense the output as it comes from Sarama on the sentinel / multi error case:

kafka: client has run out of available brokers to talk to (Is your cluster reachable?): 3 errors occurred:
        * kafka server: SASL Authentication failed.: 1 error occurred:
        * Authentication failed, invalid credentials


        * kafka server: SASL Authentication failed.: 1 error occurred:
        * Authentication failed, invalid credentials


        * kafka server: SASL Authentication failed.: 1 error occurred:
        * Authentication failed, invalid credentials

[dnwe]

Added a commit to condense the default output a little which now gives:

kafka: client has run out of available brokers to talk to (Is your cluster reachable?): 3 errors occurred:
        * kafka server: SASL Authentication failed.: Authentication failed, invalid credentials
        * kafka server: SASL Authentication failed.: Authentication failed, invalid credentials
        * kafka server: SASL Authentication failed.: Authentication failed, invalid credentials