Use the text "OK" instead of "Proceed" in updates dialog when things go wrong

Summary:
Otherwise the button says "Proceed" when the dialog is showing you an error condition
in which you cannot proceed:

Reviewers: VDG, Discover Software Store, apol, filipf

Reviewed By: VDG, filipf

Subscribers: filipf, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D24842