vulkan: check for unsupported extensions

Return VK_ERROR_EXTENSION_NOT_PRESENT when an app attempts to enable an
unsupported extension.  This fixes a regression introduced by my top
rewrite (commit 0c203249: vulkan: rewrite top of loader).

Since we do not cache HAL extensions, each vkCreateInstance or
vkCreateDevice call is preceded by two extension enumeration calls
internally.

Change-Id: I5342c1a5c9a5452dd2bc52933a5ee558db174048