Format the establishDataChannel error log message like other log mess…

…ages.

It was sticking out in the context of other log messages.

2020/04/30 22:39:10 WebRTC: DataChannel created.
2020/04/30 22:39:20 establishDataChannel: timeout waiting for DataChannel.OnOpen
2020/04/30 22:39:20 WebRTC: closing PeerConnection
2020/04/30 22:39:20 WebRTC: Closing
2020/04/30 22:39:20 WebRTC: WebRTC: Could not establish DataChannel  Retrying in 10s...

client/lib/webrtc.go

@@ -121,7 +121,7 @@ func (c *WebRTCPeer) connect(config *webrtc.Configuration, broker *BrokerChannel
 	}
 	c.transport, err = c.establishDataChannel()
 	if err != nil {
-		log.Printf("establishDataChannel: %v", err)
+		log.Printf("WebRTC: establishing data channel: %v", err)
 		// nolint: golint
 		return errors.New("WebRTC: Could not establish DataChannel")
 	}