If vkAcquireNextImageKHR fails, the aquireImageSemaphore would be leaked,
wrap it in a vk::Scoped so it is always cleaned up.

BUG=angleproject:2464

Change-Id: Ic4a0e16c89ea7c35cf060e5601760422e673c080
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1585318Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Geoff Lang <geofflang@chromium.org>