fix(mdx-loader): suppress image reading warning in Yarn PnP; log warning instead of error

[Josh-Cena]

Motivation

Fix #6774. Yarn PnP patches FS and generates a virtual file handle, but they forgot to map it back to the actual fd when calling stat(). This makes image-size unhappy. More information in yarnpkg/berry#3889 (comment).

In addition, I've downgraded the error to a warning, hoping it would be less intimidating for users.

Have you read the Contributing Guidelines on pull requests?

Yes

[netlify]

✔️ [V2]

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[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	57
🟢 Accessibility	100
🟢 Best practices	92
🟢 SEO	100
🟢 PWA	90

Lighthouse ran on https://deploy-preview-6779--docusaurus-2.netlify.app/

[github-actions]

Size Change: 0 B

Total Size: 789 kB

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	49.7 kB
website/build/assets/css/styles.********.css	106 kB
website/build/assets/js/main.********.js	596 kB
website/build/index.html	37.6 kB

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