Only save the last update timestep on success

Summary:
If the check for updates failed, it'll trigger a new check on the next timer
interval (1h) now.

Test Plan:
Ran the check with YaST open in the background to cause the check
to fail. Now the timestamp does not refresh anymore.

Reviewers: lukas, jgrulich

Subscribers: plasma-devel

Differential Revision: https://phabricator.kde.org/D9503