Change all usage of VK_ERROR_VALIDATION_FAILED_EXT to VK_ERROR_UNKNOWN or VK_ERROR_INITIALIZATION_FAILED
Change all usage of VK_ERROR_VALIDATION_FAILED_EXT to VK_ERROR_UNKNOWN or VK_ERROR_INITIALIZATION_FAILED

Compatibility breaking! Changes documentation of vmaCheckCorruption, vmaCheckPoolCorruption.
This is due to VK_ERROR_VALIDATION_FAILED_EXT coming from an extension and might not be available.
Fixes #190

GPUOpen-LibrariesAndSDKs/VulkanMemoryAllocator,GPUOpen-LibrariesAndSDKs/VulkanMemoryAllocator,GPUOpen-LibrariesAndSDKs/VulkanMemoryAllocator