dir_detect: warn on eperm

dir_detect: warn on eperm

if user has lxc.rootfs.path = /some/path/foo, but can't access
some piece of that path, then we'll get an unhelpful "failed to
mount" without any indication of the problem.

At least show that there is a permission problem.

Signed-off-by: Serge Hallyn <shallyn@cisco.com>