Vulkan Loader doesn't return an error when the driver binary couldn't be found

**Describe the bug**
After discussions with @richard-lunarg and @aitor-lunarg, I started investigating running Lavapipe and KosmicKrisp on macOS using `vk_loader_settings.json` (generated from Vulkan Configurator).

When adding drivers, eg the following vk_loader_settings.json file, the additional drivers are not found by `Vulkan Configurator` or `vulkaninfo`.

Additionally, simply copying the additional drivers in `/usr/local/share/vulkan/icd.d` next to MoltenVK doesn't work either.

**Environment (please complete the following information):**
- OS: macOS

**To Reproduce**
```
[vk_loader_settings.json](https://github.com/user-attachments/files/23125923/vk_loader_settings.json)
```

vulkaninfo --summary:
[log.txt](https://github.com/user-attachments/files/23125941/log.txt)

**Additional context**