feat(server): Log a warning for unsupported subprotocols

Closes: enisdenjo#92

src/use/ws.ts

@@ -2,6 +2,7 @@ import type * as http from 'http';
 import type * as ws from 'ws';
 import { makeServer, ServerOptions } from '../server';
 import { Disposable } from '../types';
+import { GRAPHQL_TRANSPORT_WS_PROTOCOL } from '../protocol';
 
 // for nicer documentation
 type WebSocket = typeof ws.prototype;
@@ -108,6 +109,14 @@ export function useServer(
     socket.once('close', (code, reason) => {
       if (pongWait) clearTimeout(pongWait);
       if (pingInterval) clearInterval(pingInterval);
+      if (!isProd && code === 1002) {
+        console.warn(
+          `WebSocket protocol error occured. It was most likely caused due to an ` +
+            `unsupported subprotocol "${socket.protocol}" requested by the client. ` +
+            `graphql-ws implements exclusively the "${GRAPHQL_TRANSPORT_WS_PROTOCOL}" subprotocol, ` +
+            'please make sure that the client implements it too.',
+        );
+      }
       closed(code, reason);
     });
   });