VP1HepVis: Assertion / error message fix

We were tripping the assertion in SbPolyhedronPolygonXSect::Internals::setData because it could be called with three points (n=3). But that case seems to be handled ok, so change the assertion to n>2.

We also got a bunch of errors from SbPolyhedronCons::SbPolyhedronCons because the tracking geometry can produce zero-height cones. Just ignore those cases.