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

src/mg_http_websocket.c

@@ -478,6 +478,8 @@ void mg_send_websocket_handshake3v(struct mg_connection *nc,
   }
   mg_printf(nc, "\r\n");
 
+  nc->flags |= MG_F_IS_WEBSOCKET;
+
   mbuf_free(&auth);
 }