loader: gh1681 - Restrict error in JSON
loader: gh1681 - Restrict error in JSON

Restrict the loader's JSON errors when reading ICD manifest files
to only return an overall error if there are no valid drivers.

Change-Id: I10edb1cdc7e4db9cfdc0b3595416f0614ed22867

KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers