Error message in case both port and socket are defined is not clear

When you define both socket and port for a listener you may get the following message:

2018-02-27 13:00:00 error : Listener 'Galera-Listener', for service 'Galera-Service', already have port 7890.

And MaxScale will shut down.

The actual problem should be that both connection methods are defined:

port=7890
socket=/tmp/maxscale.sock

The message is misleading so I'd suggest to use a more adequate one like:

2018-02-27 13:00:00 error : Listener 'Galera-Listener', for service 'Galera-Service', Port method already defined, can't have port and socket for the same listener.

Apparently prior to 2.2.2 this double listening method definition was not leading to a blocking error.