webdav-server: How to handle errors while reading dir

[Xuanwo]

          Please don't unwrap error returned by OpenDAL, return en error instead.

Since FsStream requires to return Box<dyn DavDirEntry>, maybe we return a bad entry instead.

This change can be done in following PRs.

Originally posted by @Xuanwo in #3119 (comment)

[Xuanwo]

Has been implemented.