Make it possible to fallback to a different icon

Summary:
If we failed to download something, it could make sense to be in control of what
icon is shown instead.

Test Plan: See D17079

Reviewers: Kirigami, mart

Reviewed By: Kirigami, mart

Subscribers: plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D17080