Allow to create new storage NotFound errors

storage/storage.go

@@ -1,6 +1,10 @@
 package storage
 
-import "github.com/play-with-docker/play-with-docker/pwd/types"
+import (
+	"fmt"
+
+	"github.com/play-with-docker/play-with-docker/pwd/types"
+)
 
 const notFound = "NotFound"
 
@@ -8,6 +12,10 @@ func NotFound(e error) bool {
 	return e.Error() == notFound
 }
 
+func NewNotFoundError() error {
+	return fmt.Errorf("%s", notFound)
+}
+
 type StorageApi interface {
 	SessionGet(string) (*types.Session, error)
 	SessionPut(*types.Session) error