## What happens

When a plugin declaring `kind: "bar"` (or a panel plugin) fails to load, the
user gets no diagnostic and the documented fallback to `omarchy.bar` does not
engage. The bar simply vanishes or silently reverts.

The cause is in `shell/shell.qml`. Both plugin Loader error handlers reference
a bare `errorString`:

```qml
// shell.qml:256 (bar option loader)
var detail = errorString && errorString() ? errorString() : ""
console.warn("bar option " + shell.activeBarId + " failed to load, falling back to " + shell.defaultBarId + ":", detail)
shell.failedBarId = shell.activeBarId
```

```qml
// shell.qml:645 (panel plugin loader)
var detail = errorString && errorString() ? errorString() : ""
if (!detail && sourceComponent) detail = sourceComponent.errorString()
```

`Loader` has no `errorString` — that member belongs to `Component`. In QML an
unqualified identifier that does not resolve throws a ReferenceError rather
than evaluating to `undefined`, so the `errorString && ...` guard cannot
short-circuit; it throws first.

Observed in the journal:

```
WARN scene: file:///home/<user>/.config/omarchy/plugins/<id>/Bar.qml[618:3]: Property value set multiple times
WARN scene: @shell.qml[256:-1]: ReferenceError: errorString is not defined
```

Two consequences at line 256:

1. The `console.warn` naming the failing plugin never runs, so nothing points
   at the plugin or the fallback.
2. `shell.failedBarId = shell.activeBarId` on the following line never runs, so
   the `activeBarId` fallback path that `failedBarId` exists to drive does not
   engage as designed.

At line 645 the same throw makes the `sourceComponent.errorString()` fallback
on the next line unreachable — which is precisely the case its own comment says
it is there to handle ("Surface both so the user sees something actionable
instead of a panel that silently refuses to open").

## Reproduce

1. Clone the stock bar: `omarchy plugin clone omarchy.bar`
2. Introduce any QML error in the clone's `Bar.qml` — e.g. add a second
   `Component.onCompleted:` to the root `Item`, which yields
   "Property value set multiple times".
3. `omarchy restart shell`

Expected: a warning naming the plugin, and a clean fallback to the built-in bar.
Actual: only the raw QML error plus `ReferenceError: errorString is not defined`;
no plugin-level diagnostic and no fallback.

## Notes

A guard that works for a possibly-undefined identifier would be
`typeof errorString !== "undefined"`, but since `Loader` does not expose the
member at all, the intended source is likely `sourceComponent.errorString()`
alone. I have not proposed a patch as the right surface is a maintainer call.

## System

- Omarchy 4.0.2-1
- Arch Linux, kernel 7.1.9-arch1-2
- Hyprland 0.56.2