feat: surface subscribeToStream polling errors

[BrightHeart23]

Summary

• invoke the optional StreamEventHandlers.onError callback when event polling fails
• normalize non-Error rejection values while retaining the existing console warning
• keep polling if the consumer's error handler throws

Validation

• npm test -- --run src/tests/events-subscribe.test.ts (11 tests passed)
• npm run typecheck
• npm run lint -- --no-fix

Closes #200

[Jaydbrown]

Thanks for the contribution here — squash-merging this now. Any follow-ups we'll track in a fresh issue. 🚀