message: Added a warning not to build empty MeshBuilders
  Inspired by
diff: @@ -443,6 +443,9 @@ impl MeshBuilder {
  /// Takes the accumulated geometry and load it into GPU memory,
  /// creating a single `Mesh`.
  + ///
  + /// Note that this returns a `GameResult<Mesh>`, since the build can fail,
  + /// for example when trying to build an empty `MeshBuilder`.
  pub fn build(&self, ctx: &mut Context) -> GameResult<Mesh> {
  Mesh::from_raw(
  ctx,
  