### What happened

When a custom/third-party bar plugin (kind `bar`, selected via `~/.config/omarchy/shell.json` or `omarchy plugin clone`) fails to load for any reason, Quickshell's shell host is supposed to fall back to the built-in `omarchy.bar` and log which plugin failed. Instead, the fallback never happens and the bar disappears entirely, because the error-handling code itself throws.

`shell.qml`'s `pluginBarLoader`:

```qml
Loader {
  id: pluginBarLoader
  ...
  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
    }
  }
}
```

`errorString` is not a bound identifier anywhere in this scope, so evaluating it throws a `ReferenceError`, which aborts the handler *before* `shell.failedBarId = shell.activeBarId` runs. Since `failedBarId` never gets set, `activeBarId` (`selectedBarId !== failedBarId && selectedBarAvailable ? selectedBarId : defaultBarId`) never falls back to `defaultBarId`, so `defaultBarLoader` never activates either. Net effect: **no bar renders at all**, instead of gracefully reverting to the stock bar.

### Steps to reproduce

1. `omarchy plugin clone omarchy.bar` (this alone is enough to trigger a load failure — see basecamp/omarchy#<omarchy-plugin-clone-required-property-issue>, filed separately)
2. `omarchy restart shell`
3. `hyprctl layers` shows no `omarchy-bar` layer surface on any level — the bar is completely gone rather than falling back to the built-in one.
4. Quickshell's log (`~/.local/state`/`XDG_RUNTIME_DIR` quickshell instance log) shows, right after the plugin's own load warnings:
   ```
   WARN scene: @shell.qml[256:-1]: ReferenceError: errorString is not defined
   ```

### Expected

The bar falls back to the built-in `omarchy.bar`, with a clear console warning naming the plugin that failed.

### Actual

No bar renders at all, and the fallback's own error-reporting code throws before it can flip `failedBarId`.

### System

- Omarchy: 4.0.0-1 (stable channel)
- Hyprland: 0.56.2
- Quickshell: 0.3.0 (quickshell-git AUR)
- `uname -a`: Linux 7.1.8-arch1-3 x86_64

### Suggested fix

Read the actual error from the `Loader` (e.g. `pluginBarLoader.errorString` if Quickshell's `Loader` type exposes it) instead of the bare, unbound `errorString` identifier — or drop the detail extraction and just log `shell.activeBarSourceUrl`.