devmapper plugin: skip plugin when not configured

This is not really an error in most cases as most people do not use
devmapper, however this shows up as an error in the logs and in the
plugin service when querying the plugins.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

snapshots/devmapper/plugin/plugin.go

@@ -20,6 +20,7 @@ package plugin
 
 import (
 	"errors"
+	"fmt"
 
 	"github.com/containerd/containerd/platforms"
 	"github.com/containerd/containerd/plugin"
@@ -40,7 +41,7 @@ func init() {
 			}
 
 			if config.PoolName == "" {
-				return nil, errors.New("devmapper not configured")
+				return nil, fmt.Errorf("devmapper not configured: %w", plugin.ErrSkipPlugin)
 			}
 
 			if config.RootPath == "" {