Cloud FS: don't propagate the error if removing a folder returns not found

for Cloud FS the folders are virtual and they, generally, disappear when the
last file is removed.

This fix doesn't work for FTP protocol for now.

Fixes #149

vfs/vfs.go

@@ -44,6 +44,7 @@ type Fs interface {
 	GetRelativePath(name string) string
 	Walk(root string, walkFn filepath.WalkFunc) error
 	Join(elem ...string) string
+	HasVirtualFolders() bool
 }
 
 var errUnsupported = errors.New("Not supported")