Stop reporting errors when no service instance is available

[dreikanter]

Summary

• Reddit post-score fetching relies on a Libreddit instance pool that has decayed to effectively a single instance. Whenever that instance flaps (downtime or rate-limiting trips the error/suspend threshold), the pool has zero operational instances and ServiceInstance.pick! raises, flooding Honeybadger (fault #130263597, 5,243 notices) for what is an expected, recurring condition.
• ServiceInstance.pick! now raises a dedicated ServiceInstance::NoInstanceAvailableError instead of a bare RuntimeError, so callers can tell an empty pool apart from a genuine fetch failure.
• RedditProcessor#score rescues NoInstanceAvailableError and silently returns 0. Real fetch errors (HTTP failures etc.) are still reported via Honeybadger.notify.

Checklist

• Updated CHANGELOG.md under Unreleased, or change is internal-only