new feature: Improve sftp error propagation so we don't get just "timeout"

### Feature Description

We are users of rustic software which have opendal as backend to access sftp. Currently, rustic does not report much about sftp/ssh errors except "timeout", [see](https://github.com/rustic-rs/rustic/issues/1488), this makes debugging ssh/sftp issues **extremely hard** and confusing. Rustic author suggested us to report the issue to your project.

Could you please improve error reporting and propagation for sftp backend up to the caller. Thanks.

### Problem and Solution

Hopefully, we get all low-level network and sftp/ssh protocol errors up with human readable message.

### Additional Context

_No response_

### Are you willing to contribute to the development of this feature?

- [ ] Yes, I am willing to contribute to the development of this feature.

Start by locating the SFTP backend and tracing where low-level SSH/SFTP and network errors become the reported "timeout". Determine the existing error-propagation path and its tests; done means callers receive human-readable underlying errors instead of only the timeout message.

Written by the indexing model from the issue text.