Improve "No internet connection!" error message

Summary:
Old message was a bit unclear. Hopefully this is better. Added an icon too to help get the message across, and because icons are pretty. :)

CCBUG: 401959

Test Plan:
Disable internet connection and launch Discover:

Reviewers: Discover Software Store, VDG, apol, leinir

Reviewed By: leinir

Subscribers: GB_2, rooty, pino, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D17598