Fixing implementation of FileJob interface in smb/sftp slaves

Summary:
This patch does the following:

1. Fixing instances of finished() being called after error() in smb slave.

2. Changing behaviour of the sftp/smb slave to conform to what is stated in D23194. In particular, the sftp slave should not call finished() in the open()/read()/write()/seek() methods.

Test Plan: It compiles...

Reviewers: chinmoyr, fvogt, sitter, dfaure

Reviewed By: dfaure

Subscribers: bcooksley, kde-frameworks-devel, kfm-devel

Tags: Dolphin, Frameworks

Differential Revision: https://phabricator.kde.org/D23207