Blender Git Loki

Scenes: forbid deleting last local scene

Previously, it was only forbidden to delete the last scene. This can
lead to the situation where a .blend file only contains linked scenes.
This is problematic, because linked data might not always be available
or can be removed from a .blend file without having an additional check
for remaining scenes.

Now there always has to be at least one local scene.

Reviewers: mont29

Differential Revision: https://developer.blender.org/D10049