no receiver unsupport notify

[JavaSCU]

if receiver do not support WebRTC when sender sent messge or file, no notify here.

in browser's console show errors like:

Uncaught TypeError: this.peers[message.to].sendText is not a function
Uncaught TypeError: this.peers[message.to].sendFiles is not a function
TypeError: Cannot read properties of undefined (reading 'sendText')
TypeError: Cannot read properties of undefined (reading 'sendFiles ')

maybe we can catch exception where sending message or file like this: /client/scripts/network.js

    _onFilesSelected(message) {
    	try {
	        this.peers[message.to].sendFiles(message.files);
	} catch(err) {
		alert("receiver may not support");   // or do something
	}
    }

    _onSendText(message) {
    	try {
		this.peers[message.to].sendText(message.text);
	} catch(err) {
		alert("receiver may not support");   // or do something
	}
    }