Allow providing an error message from the application

Summary:
Allows us keep the message provided by QQuickWindow::sceneGraphError to later have on our bug reports.
https://doc.qt.io/qt-5/qquickwindow.html#sceneGraphError
These are generally problems at the OpenGL level that the user will usually be unable to take action on.

CCBUG: 375913

Test Plan: See D27985

Reviewers: #frameworks, sitter, dfaure

Reviewed By: dfaure

Subscribers: dfaure, kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D27986