Make connection fail when connecting to non-printer port

[MultiMote]

MultiMote/niimblue-node#2 (comment)

PS: in the meantime I found another "weird" response when you send a /connect request while the printer is off and then immediately request /info, it returns weird data (empty printerInfo) - I think it would make more sense to return the usual "not connected" error if a connection attempt is pending

Lib should not successfully connect to any serial port.