kstart: Prevent crash when running without args

Summary:
Despite an error being thrown, kstart would continue, trying to
access an item that didn't exist in parser's arguments QList.

Display the help message and exit instead.

Signed-off-by: Tasos Sahanidis <tasos@tasossah.com>

Reviewers: davidedmundson, apol

Reviewed By: davidedmundson, apol

Subscribers: apol, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22613