### What happened

When a custom bar option fails to load, the shell is supposed to fall back to the default bar. Instead the error handler itself throws, the fallback never runs, and the session is left with **no bar at all** — the layer surface is gone and windows expand into the reclaimed space.

From the quickshell log (Omarchy 4.0.1-1):

```
WARN scene: @shell.qml[256:-1]: ReferenceError: errorString is not defined
```

### Why

`shell/shell.qml`, `pluginBarLoader.onStatusChanged` (line 257 on master):

```qml
onStatusChanged: {
  if (status === Loader.Error) {
    var detail = errorString && errorString() ? errorString() : ""
    console.warn("bar option " + shell.activeBarId + " failed to load, falling back to " + shell.defaultBarId + ":", detail)
    shell.failedBarId = shell.activeBarId
  }
}
```

`QtQuick.Loader` has no `errorString` member, and in QML an unqualified lookup of an undefined name throws `ReferenceError` — the `errorString &&` guard does not protect against that. The throw aborts the handler, so:

- the diagnostic `console.warn` never prints (the real load error is swallowed), and
- `shell.failedBarId` is never set, so `activeBarId` (line 181) never resolves back to `defaultBarId`, so `defaultBarLoader` never activates.

The same unguarded `errorString` pattern also appears at line 646 in the plugin-widget loader path and looks susceptible to the same failure.

### Reproduce

1. Switch to any bar option that fails to load — e.g. the verbatim clone from #9115, or `omarchy bar use x.bar` pointing at a bar QML with a syntax error.
2. `omarchy restart shell`
3. No bar on any monitor; log shows the ReferenceError above instead of the intended "falling back to omarchy.bar" warning.

### Expected

The failed bar option is recorded in `failedBarId` and the stock bar loads, as the surrounding code clearly intends.

### Possible fix direction

Guard the lookup (`typeof errorString === "function"`) or take the detail from the loader's `sourceComponent` when available — and consider wrapping the handler so a failure in diagnostics can never block `shell.failedBarId = shell.activeBarId`.

### System

- Omarchy 4.0.1-1, Arch Linux (kernel 7.1.9-arch1-2)
- CPU: Intel i7-10700K, GPU: NVIDIA Quadro P2200
- Verified the same code is on current master (`shell/shell.qml` lines 257 and 646)