fix(test): surface merge reporter errors

[mturac]

Fixes #40983

Summary

• keep reporter-thrown errors separate from normal reported test errors
• print reporter errors when merge-reports fails because a reporter throws

Tests

• npm run build
• npm run ttest -- tests/playwright-test/reporter-blob.spec.ts --grep "should fail merge and report error when reporter throws in onEnd"
• npx eslint packages/playwright/src/reporters/merge.ts packages/playwright/src/reporters/multiplexer.ts
• npm run tsc -- --pretty false
• git diff --check