Add a scanner error check to ClusterCounter.Count.

It was silently exiting at the "recordingStart":"2022-09-23T17:06:59.680537075Z"
line, the first line whose length (66873) exceeds
bufio.MaxScanTokenSize. Now distinctcounter exits with an error status
instead of reporting partial results.

$ ./distinctcounter -from 2023-01-01T00:00:00Z -to 2023-01-10T00:00:00Z -in metrics-ip-salted.jsonl
2023/04/20 13:54:11 unable to count:bufio.Scanner: token too long

common/ipsetsink/sinkcluster/reader.go

@@ -55,6 +55,10 @@ func (c ClusterCounter) Count(reader io.Reader) (*ClusterCountResult, error) {
 			return nil, err
 		}
 	}
+	err = inputScanner.Err()
+	if err != nil {
+		return nil, err
+	}
 	result.Sum = counter.Count()
 	return &result, nil
 }