consumer: return UnknownTopicID even though it is retryable

See the embedded comment.

Closes #357.

pkg/kgo/source.go

@@ -854,6 +854,14 @@ func (s *source) handleReqResp(br *broker, req *fetchRequest, resp *kmsg.FetchRe
 					keep = true
 				}
 
+			case kerr.UnknownTopicID:
+				// We need to keep UnknownTopicID even though it is
+				// retryable, because encountering this error means
+				// the topic has been recreated and we will never
+				// consume the topic again anymore. This is an error
+				// worth bubbling up.
+				keep = true
+
 			case kerr.OffsetOutOfRange:
 				// If we are out of range, we reset to what we can.
 				// With Kafka >= 2.1, we should only get offset out