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
bc788929aa