Improve error handling in file ioslave

Summary:
When executing an action with elevated privileges file ioslave directly checks errno
and then decides whether to continue or not. Since errno can change frequently
between function call it should not be relied upon.
With this patch after a function fails errno value is saved and then that saved value
is used wherever required.

Reviewers: dfaure

Reviewed By: dfaure

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D10266