Don't set log_min_messages from the debugger connections, this was
stopping the non-superuser to debug the code.

Instead set client_min_messages to error for the listener connection, that will
avoid any unwanted messages to appear in message window from the listener. And,
shows the messages from the invoker only. Reported by Guillaume Lelarge

(cherry picked from commit e551e08922a7f940af1968c9df15ee5ded5cfb28)