If the resulting geometry from applying a geometry nodes modifier contains no mesh, give an error message. This gives people something to search and makes the behavior more purposeful. Also remove the `modifyMesh` implementation from the geometry nodes modifier, since it isn't necessary anymore. And remove the existing "Modifier returned error, skipping apply" message which was cryptic and redundant if applying returns an actual error message. Resolves T103229 Differential Revision: https://developer.blender.org/D16782