add error handling docs

Author: stefangutica

docs/sdk-and-tools/rest-api/ws-subscriptions.md

@@ -364,13 +364,60 @@ main().catch(console.error);
 
 ---
 
+## Error Handling
+
+Unexpected behaviors, such as sending an invalid payload or exceeding the server's subscription limits, will trigger an `error` event emitted by the server.
+
+You should listen to this event to handle failures gracefully.
+
+### Payload (DTO)
+
+The error object contains context about which subscription failed and why.
+
+| Field   | Type   | Description                                                                        |
+|---------|--------|------------------------------------------------------------------------------------|
+| pattern | string | The subscription topic (event name) that was requested (e.g., `subscribeTransactions`). |
+| data    | object | The original payload sent by the client that caused the error.                       |
+| error   | object | The specific error returned by the server.                                 |
+
+### Example usage
+
+```js
+import { io } from "socket.io-client";
+
+// ... setup socket connection ...
+
+// Listen for generic errors from the server
+socket.on("error", (errorData) => {
+  console.error("Received error from server:");
+  console.dir(errorData, { depth: null });
+});
+```
+
+### Error Example
+
+**Scenario:** The client attempts to open more subscriptions than the server allows (e.g., limit of X).
+
+```json
+{
+  "pattern": "subscribeCustomTransactions",
+  "data": { 
+    "function": "newGame" 
+  },
+  "error": "Maximum number of X global subscriptions accepted by server reached!"
+}
+```
+
+---
+
 ## Summary
 
 - WebSocket endpoint is dynamically obtained via `/websocket/config`.
 - Each stream has its own subscribe and update events.
 - Payload DTOs define allowed fields and required/optional rules.
 - Update messages mirror REST API and include `<resource>Count` fields.
 - `<resource>Count` reflects **total items at the moment of update**.
+- Errors are emitted via the standard `error` event, containing the pattern, original data, and error message.
 - Uses `socket.io-client`.
 
-This document contains everything required to use MultiversX WebSocket Subscriptions effectively.
+This document contains everything required to use MultiversX WebSocket Subscriptions effectively.