Fail build on webpack warning
Fail build on webpack warning

Fixes: CNVS-38653

This will prevent compile_assets from continuing
if webpack had any warnings.

Test plan:
* Comment out `new webpack.IgnorePlugin(/\.flow$/),`
  in frontend_build/baseWebpackConfig.js
* run webpack
* it should return a non-zero exit code

Change-Id: Iecfd47bc4e83e15a288397e102716535a5a4ef0d
Reviewed-on: https://gerrit.instructure.com/122994
Reviewed-by: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
Reviewed-by: brian kirkby <9a8a656cc2d0f066b1a373934e1a6a0652844ecd@instructure.com>
Tested-by: Jenkins
Product-Review: Ryan Shaw <ea3cd978650417470535f3a4725b6b5042a6ab59@instructure.com>
QA-Review: Ryan Shaw <ea3cd978650417470535f3a4725b6b5042a6ab59@instructure.com>

instructure/canvas-lms,instructure/canvas-lms,venturehive/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,venturehive/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,venturehive/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,venturehive/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,djbender/canvas-lms