Reply a meaningful dBus error when detecting unmanaged windows

Currently, the method `createErrorReply()` with empty strings
errors out and the client gets no answer:
```
QDBusConnection: error: could not send error message to service "":
Error name cannot be empty
```

Simple test:
qdbus org.kde.KWin /KWin org.kde.KWin.showDebugConsole
qdbus org.kde.KWin /KWin org.kde.KWin.queryWindowInfo

Error name uses D-Bus Interface-like notation as suggested in
https://doc.qt.io/qt-5/qdbuserror.html#name

CCBUG: 340477