Show up warning when there are unhandled exception during Export process
When there are program exception/errors occur during Export Model (moc3) process, it wasn't clearly shown because Live2D just silently logs the error in Log panel instead.
Most often this caused confusion with users because they don't see the exported files, and then tried to export many times without realizing there are something wrong with the project.
Since Live2D 5.0 is all about UI/UX improvement, I would like to see a warning dialog when there are any errors in I/O operations like saving or exporting.
Ideally, it should show the stacks as well, like .NET exception dialog.