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

common/connection.go

@@ -252,6 +252,10 @@ func (c *BaseConnection) RemoveDir(fsPath, virtualPath string) error {
 	var fi os.FileInfo
 	var err error
 	if fi, err = c.Fs.Lstat(fsPath); err != nil {
+		// see #149
+		if c.Fs.IsNotExist(err) && c.Fs.HasVirtualFolders() {
+			return nil
+		}
 		c.Log(logger.LevelWarn, "failed to remove a dir %#v: stat error: %+v", fsPath, err)
 		return c.GetFsError(err)
 	}