Fix: Show a better error messages when project is not user-maanged

[kfirstri]

Summary

When the appID configured inside .app.jsonc file is not a user-managed project we get an ugly APIError with json data.

1. Figure out why it's not showing the formatted message
2. Format the message
3. Add hints regarding create / eject command

https://us.posthog.com/project/211871/error_tracking/019c34ee-6c6a-72e0-aa99-8b9b60c5404c?timestamp=2026-02-06T21%3A55%3A23.203000Z

Error deploying functions: { "message": "This endpoint is only available for apps with user-managed source code", "detail": null, "traceback": null, "extra_data": { "app_id": "69865a4ce771b575b92e5ff1", "reason": "managed_source_code_app" } }