Add logging when max frame size is exceeded

When gateway is proxying websocket connections, if a message exceeds the max frame size, the websocket connection is dropped.

(This is related to #163)

However, there's no logging that indicates this has happened, even with TRACE level logging enabled in gateway.

It'd be very helpful if there was a warning message logged within the gateway that indicates this has happened. (Specifically, that the message exceeded the maxFrameSize, so the messages was dropped, and the websocket connection was terminated).