[Explorer] Integration Connections reports unregistered connector driver only to the console, not the user

## Summary

When an Integration Connection references a driver class that has no registered connector, the Integration Connections UI surfaces the failure only as a raw `console.error` — the page gives the user no indication that the listing failed.

```
console.error: [IntegrationConnections] ListSourceObjects failed: No connector registered for
driver class "BettyConnector". Ensure a class is decorated with
@RegisterClass(BaseIntegrationConnector, 'BettyConnector').
```

The message itself is good — it names the class and the exact decorator needed. The problem is that it goes to the console instead of to the user, so from the UI the operation just appears to do nothing.

## Impact

Low. Observed 2× in regression run `run-20260727T202606Z` on test T120 (Integrations). The test did not fail on it.

This is a UX/error-surfacing gap rather than a functional defect: a misconfigured or not-yet-installed connector is a legitimate state, and it should render as an explanatory empty/error state on the connections page — ideally repeating the "register a class decorated with `@RegisterClass(BaseIntegrationConnector, '')`" guidance where the user can see it.

## Note on the specific driver

`BettyConnector` appears to be demo/seed data in the regression database with no corresponding registered connector class. Whether that seed row should exist is a separate question from whether the UI should degrade gracefully when it does — this issue is about the latter.

---
*Found by the MJ Explorer Computer Use regression suite, run `run-20260727T202606Z` (2026-07-27).*

Start at the Integration Connections UI path that calls ListSourceObjects, using regression test T120 and run run-20260727T202606Z as the reported reproduction context. Trace how the existing console.error is handled and identify the page's empty or error-state entry point. Done means the unregistered-driver failure is visible in the connections page with the existing registration guidance, rather than only in the console.

Written by the indexing model from the issue text.