Error bar trace ordering

[cpsievert]

Discovered via plotly/plotly.R#513

Here is a MRE http://codepen.io/cpsievert/pen/NNddEa

[etpinard]

A side effect from our per-trace type <svg> layer ordering.

Error bar <g>s should really be part of their corresponding <g class="scatterlayer"> and/or <g class="barlayer"> to avoid this effect.