Refactor a bit PullSubscription.fetch

[orsinium]

1. Reuse the same code for pulling from the queue in both _fetch_one and _fetch_n.
2. Don't get from queue more than batch messages.
3. Refactor this loop a bit: don't check if the queue is empty before pulling from it. Instead, try pulling and exiting on the QueueEmpty exception raised. "Ask for forgiveness, not permission". And since before it had except Exception: pass, it used to have 3 function calls instead of one now (because it was going on the next iteration loop on failure instead of exiting the loop immediately).
4. Drop except Exception: pass, we don't want to suppress all ranges of exceptions when pulling from the queue.