Use errors.Cause() when looking at storage errors

The storage library uses github.com/pkg/errors to wrap errors that it
returns from many of its functions, so when passing them to
os.IsNotExist() or comparing them to specific errors defined in the
storage library, unwrap them using errors.Cause().

Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>

libkpod/container_server.go

@@ -20,6 +20,7 @@ import (
 	"github.com/kubernetes-incubator/cri-o/pkg/storage"
 	rspec "github.com/opencontainers/runtime-spec/specs-go"
 	"github.com/opencontainers/selinux/go-selinux/label"
+	"github.com/pkg/errors"
 	pb "k8s.io/kubernetes/pkg/kubelet/api/v1alpha1/runtime"
 )
 
@@ -156,7 +157,7 @@ func (c *ContainerServer) Update() error {
 	defer c.updateLock.Unlock()
 
 	containers, err := c.store.Containers()
-	if err != nil && !os.IsNotExist(err) {
+	if err != nil && !os.IsNotExist(errors.Cause(err)) {
 		logrus.Warnf("could not read containers and sandboxes: %v", err)
 		return err
 	}