demos: Print XCB errors to stderr
demos: Print XCB errors to stderr

Printing to stdout is weird.

Change-Id: If53e8708b853989ae8a9ea61fa4d57f27b24b45c

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