client: collapse shard errors

When sharding requests, we previously would return a unique error
message per missing topic / partition / leader. This would show up as a
bunch of shards. While the errors were informative, they aren't really
any more actionable than one slightly less informative error. We also
don't need a dedicated error message, because we split the request
appropriately such that the user can tell what all topics / partitions
failed from the split request in the shard.

pkg/kgo/client.go

@@ -1631,23 +1631,29 @@ func (cl *Client) fetchMappedMetadata(ctx context.Context, topics []string) (map
  return mapping, nil
 }
 
+var (
+ errMissingTopic = errors.New("topic was not returned when looking up its metadata")
+ errMissingPartition = errors.New("partition was not returned when looking up its metadata")
+ errNoLeader = errors.New("partition has no leader from metadata lookup")
+)
+
 func missingOrCodeT(t string, exists bool, code int16) error {
  if !exists {
- return fmt.Errorf("topic %s was not returned when looking up metadata for the topic", t)
+ return errMissingTopic
  }
  return kerr.ErrorForCode(code)
 }
 
 func missingOrCodeP(t string, p int32, exists bool, code int16) error {
  if !exists {
- return fmt.Errorf("topic %s partition %d was not returned when looking up metadata for the topic", t, p)
+ return errMissingPartition
  }
  return kerr.ErrorForCode(code)
 }
 
 func noLeader(t string, p int32, l int32) error {
  if l < 0 {
- return fmt.Errorf("topic %s partition %d has no leader according to the metadata lookup", t, p)
+ return errNoLeader
  }
  return nil
 }