QQmlError.object now holds the scope object that caused exceptions.

Exception errors sent via QQmlEngine::warnings lacked a pointer to the
containing scope. I added an object property to QQmlError, and the
necessary code to fill it with the QObject* scope from binding exception
callbacks.

Task-number: QTBUG-30930
Change-Id: I2a987e8cefc3a2a474d338893e9ebcb77c167adf
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
Reviewed-by: Alan Alpert <aalpert@blackberry.com>