STLWriter: Raise an error for non-triangles

The STL format only supports triangles and previously, it just truncated
polygons silently. Cause an error to occur when this happens so that
users know.

Change-Id: I9c2f96d887074943130a8442d6796cb0c2ce1bbf
Merge topic 'stl-writer-errors' into master

9a00db82 STLWriter: Raise an error for non-triangles