Inform the client when a SASL message cannot be sent

Inform the client when a SASL message cannot be sent

When SASL is properly configured with a 'target' server, we are able
to inform the client when the message fails to send.
Currently if a target is configured and it is offline, no response is
sent. This can cause some clients to time out while waiting for a response.

If a target isn't configured, behaviour will not change with this commit.
The default of '*' will still send to all servers.
Updated example config with 'target' variable.