[Scripting] Throw a error from the Containment when the widget is not found

Summary:
Let's also move the applet pointer to right before it's used and
remove duplicated declarations.

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D6424