fix: return 404 for nonexistent question with api scraper

[vlnst]

With SCRAPER=api, a nonexistent or deleted question ID (e.g. /questions/2120178) panics with index out of range [0] with length 0 and returns a 500.

Cause: the API returns {"items":[]} for such questions, and getQuestionContent indexes Results[0] without checking.

Fix: return ErrQuestionNotFound when items is empty; ViewQuestion renders the error page with 404 for it.

Testing: added a unit test with a stubbed transport. Locally, /questions/2120178 returns 404 and /questions/11227809 returns 200.