Major Description
Unsupported SFTP message types should just respond with SSH_FX_OP_UNSUPPORTED, and not also throw an exception, since that ends the session. When I use Ubuntu's Nautilus integration for SFTP that attempts READLINK requests quite a lot, which are currently unimplemented in SftpSubsytem. If we stop throwing the exception and just return the unsupported response, it works fine.
Attaching a patch.