Qt Lottie Animation: Provide a way to query the error

If we get a LottieAnimation.Error (https://doc.qt.io/qt-6/qml-qt-labs-lottieqt-lottieanimation.html#status-prop), we have no way of finding out what exactly the error is.

Suggestion

Add a method or property called errorString to provide human-readable information about the error, such as "File not found".

For example:

• https://doc.qt.io/qt-6/qcamera.html#errorString-prop
• https://doc.qt.io/qt-6/qml-qtqml-component.html#errorString-method