code: func (tx *transaction) fetchBlockRow(hash *chainhash.Hash) ([]byte, error) {
  	blockRow := tx.blockIdxBucket.Get(hash[:])
  	if blockRow == nil {
  		str := fmt.Sprintf("block %s does not exist", hash)
  		return nil, makeDbErr(database.ErrBlockNotFound, str, nil)
  	}
  
  	return blockRow, nil
  }
docs: // fetchBlockRow fetches the metadata stored in the block index for the provided
  // hash.  It will return ErrBlockNotFound if there is no entry.