errors returned by admin DeleteRecords are inconsistent

[dnwe]

RE: admin.go#L576-L623

1. it sometimes returns 1 single Kerror when a partition leader could not be found (and ignores the requests to all the other partitions), while sometimes it will return ErrDeleteRecords with any per partition errors wrapped. It should really consistently return 1 type of error
2. errs = append(errs, errors.New(deleteRecordsResponsePartition.Err.Error()))
   when it wraps the Kerror from each partition, it creates a new error from the string which makes the error type checking difficult. I would expect it to preserve the Kerror type.