Fix T57660: Export of STL format causes an exception.

Raising StopIteration was never a good idea to abort a generator
function, and since 3.6 it's converted to a regular RuntimeError
exception!

Proper way to do that is just to use return statement (as long as there
is a yield statement somewhere in the function code, it is a generator,
and return statement has proper special handling in that case).

io_mesh_stl/blender_utils.py

@@ -84,7 +84,7 @@ def faces_from_mesh(ob, global_matrix, use_mesh_modifiers=False):
     try:
         mesh = ob.to_mesh(bpy.context.depsgraph, use_mesh_modifiers)
     except RuntimeError:
-        raise StopIteration
+        return
 
     mat = global_matrix @ ob.matrix_world
     mesh.transform(mat)