WebSocketServerTransport suffers an unhandled exception on invalid JSON

[orlenko]

Describe the bug
Sending an invalid JSON string to WebSocketServerTransport causes the server to crash, because of an unhandled exception.

To Reproduce
Steps to reproduce the behavior:

1. Create a basic application with WebSocketServerTransport, as described in the main README.
2. Start the server
3. Open a websocket from a browser, e.g. const ws = new WebSocket('ws://localhost:3331/');
4. Send a string that is not valid JSON, e.g. ws.send('hi');
5. The server crashes with an error.

Expected behavior
The server should respond with error code -32700, as per the specification.

[orlenko]

Proposed fix: #749