Show that the last check failed if no updates available

Summary:
If the last check for updates failed, it would be misleading to show that
no updates are available.

Test Plan:
Ran pkupdates while YaST is open, instead of (incorrectly) showing
that no updates are available, it shows that the last check failed.

Reviewers: lukas, jgrulich

Subscribers: plasma-devel

Differential Revision: https://phabricator.kde.org/D9504