Blender Git Loki

Fix T64827 (part two): "Batch-Generate Previews" fails with certain files

Some values (not properly handled by do_versions?) could cause
'rna_backup_restore()' to throw an error resulting in the whole preview
generation not being saved to the .blend file.

I've checked folowwing demo files:
- race_spaceship.blend
- wasp_bot.blend

Here the offending seetings were:
- `UnitSettings` (`length_unit`, ...)
- `FFmpegSettings` (`ffmpeg_preset`, ...)

For now, these are now excluded in `exclude_props`

Reviewers: brecht

Differential Revision: https://developer.blender.org/D5255