fix(cli): always show error stack to unhandled rejection

[Josh-Cena]

Motivation

Currently, all build-time errors don't show proper stack traces because we only log the error itself, which serializes to the message. We should log the full stack instead.

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Throw an error from lib/commands/build.js.

Before:

After:

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[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	66
🟢 Accessibility	100
🟢 Best practices	92
🟢 SEO	100
🟢 PWA	90

Lighthouse ran on https://deploy-preview-7218--docusaurus-2.netlify.app/

[github-actions]

Size Change: 0 B

Total Size: 799 kB

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	50 kB
website/build/assets/css/styles.********.css	106 kB
website/build/assets/js/main.********.js	604 kB
website/build/index.html	38.8 kB

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