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
b1494bcea7