libvulkan: Don't log error if layer dir doesn't exist

It's normal for the global layer dir to not exist, and would be a
serious system bug if the APK native library dir didn't exist. Either
way, a VERBOSE message will be seen by the appropriate people, without
spamming the log during normal operation.

Change-Id: I7419b46304bb98b4dbb6d02effff0a4062c6de79
(cherry picked from commit e0059ade3836ec778f16a7c8998d056a21d2a7ab)