Fix: ESLint 9+ path: handle child script errors in whitespace-async.js

[dataCenter430]

Summary

When CLIEngine is not available (ESLint 9+), whitespace.js loads config by running whitespace-async.js with execSync and parsing its stdout as JSON. The async script had no .catch() on its promise, so rejections became unhandled: the child could exit non-zero and print to stderr while the parent received empty or partial stdout and failed with a generic JSON.parse error, hiding the real failure.

Approach

• Do not add try/catch in whitespace.js; errors should still crash.
• Fix the child script so the ESLint 9/10 path fails in a clear, diagnosable way.

Changes

• packages/eslint-config-airbnb-base/whitespace-async.js: Add .catch() so that on rejection we log the error to stderr and process.exit(1).
• packages/eslint-config-airbnb/whitespace-async.js: Same change.

On success, only JSON is written to stdout, so the parent’s execSync + JSON.parse continues to work. On failure, the real error goes to stderr and the process exits 1, so execSync throws and the thrown error includes the child’s stderr, making the root cause visible without masking crashes.

Fixes #3238