Summary

This pull request fixes the unhandled promise rejection in whitespace-async.js when the ESLint async path is used.

Problem

onlyErrorOnRules(...) returns a Promise, but the current implementation only handles the fulfilled case using .then(). If the Promise rejects, the rejection becomes unhandled, which may cause the child process to exit unexpectedly or produce unclear error messages.

Solution

• Add a .catch() handler to the Promise chain.
• Log the error using console.error().
• Set process.exitCode = 1 so the process exits with a non-zero status instead of leaving the rejection unhandled.

Testing

• Verified that successful execution is unaffected.
• Verified that rejected Promises are handled gracefully.
• Confirmed that the process exits with code 1 instead of producing an unhandled promise rejection warning.

Fixes #3237