sftp: map sftp_open error to kio error

Summary:
sftp gives out more relevant errors such as SSH_FX_PERMISSION_DENIED,
let's forward them as KIO errors instead of using the general cannot open
error.

Test Plan: hoping for the best! bad permissions on the remote now actually raise suitable kio errors, functionally that seems to make little difference unfortunately

Reviewers: feverfew

Reviewed By: feverfew

Subscribers: kde-frameworks-devel, kfm-devel

Tags: #dolphin, #frameworks

Differential Revision: https://phabricator.kde.org/D29743