Fail build when Webpack produces warnings or errors

[DreierF]

Fixes #1527

Changes

This change makes the Snowpack build fail when Webpack produces errors.
It also adds an option to fail the build on warnings as well as that also often indicates some misconfiguration. Failure on warnings is however optional and is disabled by default to be non-breaking. I was not sure here whether this should be the default.

Testing

I tested it manually by linking the plugin against a snowpack build that succeeded despite having errors.
However the error message I returned in the reject method does not show up in the log. Is this expected?

Docs

I updated the docs of the plugin to reflect the new option.

[vercel]

This pull request is being automatically deployed with Vercel (learn more).
To see the status of your deployment, click below or on the icon next to each commit.

🔍 Inspect: https://vercel.com/pikapkg/snowpack/pkq89zsxa
✅ Preview: https://snowpack-git-failwebpackonerror.pikapkg.vercel.app

[FredKSchott]

+1 conceptually! Added two small comments on implementation

[FredKSchott]

No worries about the master branch, I believe those are outdated commits from back when we were juggling the old and new docs sites. We'll delete that branch as soon as all PRs are no longer targeting it.

[FredKSchott]

Thanks!

[melissamcewen]

Thank you so much!