Fix handling of WS handshake error response

Check response code, make sure it's 101.
Pass http_message to the client to keep it appraised.
This represents a slight change in the API -
in case of an error MG_EV_WEBSOCKET_HANDSHAKE_DONE will now be delivered where previosuly connection would just hang.
Clients that do not examine the argument may for a moment think handshake has succeeded but in fact connection will be closed immediately.

CL: mg: Fix handling of WS handshake error response

PUBLISHED_FROM=645a43d9e5bee216e54411f85827c9b974e9a7d1

docs/c-api/mg_http.h/mg_set_protocol_http_websocket.md

@@ -30,7 +30,9 @@ The user-defined event handler will receive following extra events:
 - MG_EV_WEBSOCKET_HANDSHAKE_REQUEST: server has received the WebSocket
   handshake request. `ev_data` contains parsed HTTP request.
 - MG_EV_WEBSOCKET_HANDSHAKE_DONE: server has completed the WebSocket
-  handshake. `ev_data` is `NULL`.
+  handshake. `ev_data` is a `struct http_message` containing the
+  client's request (server mode) or server's response (client).
+  In client mode handler can examine `resp_code`, which should be 101.
 - MG_EV_WEBSOCKET_FRAME: new WebSocket frame has arrived. `ev_data` is
   `struct websocket_message *`