new-counter-demo web UI reports success for failed KubeEdge API responses

**What happened**:

The new-counter-demo web UI handlers treat every response without a transport error as successful. If the KubeEdge API returns an HTTP error such as 404 or 500, `GetStatus`, `SetStatus`, `ResetCount`, and `SetCount` still return a successful JSON response to the browser.

**What I expected to happen**:

The web UI should detect non-2xx upstream responses and report an upstream failure instead of claiming the requested operation succeeded.

**How to reproduce it (as minimally and precisely as possible)**:

1. Run the web UI while the configured KubeEdge API endpoint returns a non-2xx response.
2. Call one of the counter status or mutation endpoints.
3. Observe that the web UI returns HTTP 200 with `success: true` or a status value.

A focused test covers the shared 2xx status classification used by all handlers.

**Anything else we need to know?**:

The change only validates the upstream status code and leaves successful response decoding and payloads unchanged.