Highlight missing backup files

[Te-k]

In some cases, you can check an incomplete backup, in that case it would be great to highlight files that should be in the backup (exist in the Manifest) but are not in the folder. I have for instance done that with c5ccaef for the SafariBrowserState module, but it should be possible to have this done automatically in the base class. I am also not sure if it should be an error or just an info.