Add clear error message for subcription attempts
Add clear error message for subcription attempts

Since we do not support subscriptions let's log attempts
and send a human-oriented error message.

webmasterkai/signalk-server-node,sbender9/signalk-server-node,SignalK/signalk-server-node,sbender9/signalk-server-node,webmasterkai/signalk-server-node,SignalK/signalk-server-node,SignalK/signalk-server-node,sbender9/signalk-server-node,SignalK/signalk-server-node,webmasterkai/signalk-server-node