snapshots: return error if readSnapshot fails
snapshots: return error if readSnapshot fails

Signed-off-by: Wei Fu <df4176d36abdc6fd0d966de5d5471522668de494@gmail.com>

mikebrow/containerd,estesp/containerd,thaJeztah/containerd,dmcgowan/containerd,mikebrow/containerd,crosbymichael/containerd,vdemeester/containerd,dmcgowan/containerd,vdemeester/containerd,estesp/containerd,thaJeztah/containerd,containerd/containerd,crosbymichael/containerd,containerd/containerd