[Bug] data[type=scattergl ] gives error if webGL is not supported

[mafar]

When browser does not have webGL enabled, type=scattergl gives error on console.
See error below

Steps To Reproduce:
I used firefox => about:config => webgl.disabled: true to disable webGL to reproduce this issue.

Expected Behaviour:
Instead of error , plotly should display following message just like type=scatter3d or type=isosurface

WebGL is not supported by your browser - visit https://get.webgl.org for more info

(disable webGL first) (Correct Behaviour scatter3d)
https://codepen.io/mafar/pen/ZEbQLZm?editors=1010

Incorrect Behaviour: scattergl
(disable webGL first)
https://codepen.io/mafar/pen/XWmXpoO?editors=1010

Comparison:

[mafar]

Dear Devs,
This Patch seems to fix the issue. Please review !!!

[archmoj]

Closed via #4777.