The case of subtracing a plane is handled specially as the
plane is not manifold, but the library has a TrimByPlane function.
The special handling code did not deal with the empty result case
properly.
Also, there was no error return checking from Manifold in this special
case, so that was added.
Also, the general error handling just assumed that any Manifold error
on the inputs was a "not manifold" error, which while probably true,
should not be assumed, so that was fixed too.
248529d6e0