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:
{F7642500, size=full}

Reviewers: #vdg, #discover_software_store, apol, filipf

Reviewed By: #vdg, filipf

Subscribers: filipf, plasma-devel

Tags: #plasma

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