Fix createSession error handling to throw errors instead of returning fallback codes

[dillonvargo]

Fix createSession error handling to throw errors instead of returning fallback codes

Summary

Fixed a critical issue in the createSession function where it would return a random fallback code even when session storage completely failed. This could lead to users thinking they successfully created a session when the data wasn't actually stored anywhere.

Key changes:

• Added session storage validation to ensure sessions are only considered successful when actually stored
• Changed error handling from returning random fallback codes to throwing proper errors
• Maintains dual storage approach (localStorage + window.matchMunchSessions fallback)

Review & Testing Checklist for Human

Risk Level: 🟡 Medium - Behavior change in error handling that needs end-to-end verification

• Test session creation flow end-to-end - Create sessions and verify they work properly, especially when localStorage might be unavailable
• Test error scenarios - Verify that session creation failures are properly displayed to users and don't result in broken states
• Verify storage validation logic - Check that the validation correctly identifies when sessions are successfully stored vs. when they fail

Test Plan

1. Test normal session creation flow (should work unchanged)
2. Test with localStorage disabled/unavailable (should fall back to memory storage)
3. Test with both storage methods unavailable (should show proper error to user)
4. Verify that existing error handling in HomeScreen.js displays appropriate error messages

Notes

• The existing error handling in HomeScreen.js (lines 117-126) appears designed to handle thrown errors, but should be verified
• No existing tests were found in the codebase, so manual testing is essential
• Session requested by: Dillon Vargo (@dillonvargo)
• Link to Devin run: https://app.devin.ai/sessions/939f6016012e4a4a816212be9fb029ac

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