Boost: Add check for css gen library before generation attempt

[dilirity]

Fixes #37091

Proposed changes:

• Add a check before trying to use library;
• Add track event if there's something wrong with the library;
• Show a friendly error if something's wrong with the library:

Critical CSS Generator library is either not found or invalid.

Before

Now

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

• Setup free Boost;
• Dequeue the critical css gen library (comment out this line - https://github.com/Automattic/jetpack/pull/37118/files#diff-a880940bee6572ef92319fcf362097099c74ab3ea6e03ba99fd6957407dd8abdR108);
• Enable critical css and try to generate. You should see the updated error.

[github-actions]

Thank you for your PR!

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Boost plugin:

• Next scheduled release: June 4, 2024.
• Scheduled code freeze: May 27, 2024.

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[haqadn]

👍🏼