scatter3d error bars not displayed unless `error_z` is defined

[nicolaskruchten]

If you look at this Pen https://codepen.io/nicolaskruchten/pen/QBZaYE you'll see two problems:

1. Error bars don't display unless error_z is there (i.e. just x or y or both gives broken output)
2. thickness doesn't seem to do anything

This seems to be true for the latest version as well as 1.38.0, which I randomly tested.

[etpinard]

cc @antoinerg

[antoinerg]

PR #3011 doesn't fix the problem with thickness. The problem lies in gl-error3d so I opened an issue over there: gl-vis/gl-error3d#5.

I also open a separate issue to deal with the thickness parameter: #3014.