Avoid losing data when saving versions

If there is an error while saving the Qt versions registry, there is a chance that some/all of the data is lost. This is because the saving procedure deletes the Qt versions registry without any backup. Also, this failure occurs silently, so the user is not aware of it until trying to create or build a project.

In case of failure, the state of the Qt versions registry should not change, and an error message should be shown to the user.