[shell] Show kactivies warning only on error

Summary:
load() is called a few times and different states. After the change I
also get the warning on a regular load.

This patch returns early whilst we're in the loading state, but still
prints the warning should kactivitymanagerd fail.

Reviewers: #plasma, mart

Reviewed By: #plasma, mart

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D18922