Use common unmount util func for TearDownAt()

pkg/volume/util/util.go

@@ -98,7 +98,7 @@ func UnmountPath(mountPath string, mounter mount.Interface) error {
 		glog.V(4).Infof("%q is unmounted, deleting the directory", mountPath)
 		return os.Remove(mountPath)
 	}
-	return nil
+	return fmt.Errorf("Failed to unmount path %v", mountPath)
 }
 
 // PathExists returns true if the specified path exists.