code: def get_block_info(self):
          
          if not self.finished:
              raise Exception("Not finished downloading")
  
          ret = []
          for (block_hash, block_data) in self.block_info.items():
              ret.append( (block_data['height'], block_data['txns']) )
  
          return ret
docs: Get the retrieved block information.
          Return [(height, [txs])] on success, ordered on height
          Raise if not finished downloading