Blender Git Loki

Raise error when reading Alembic with degenerate geometry, cherry picked from tangent anim blender278 remote

Raise error when reading Alembic with degenerate geometry Alembic files containing faces with fewer than 3 vertices were causing Blender to crash. While this is not a proper fix (would require actually handling the invalid geometry), this at least prevents the crash, instead giving an error message, informing of the degenerate geometry. (cherry picked from commit aeb4fdbe3ef63a3fcf7920f5fb1f4484da1a1fe3)