do not show error info if prepare header failed as have no ticket

consensus/datong/consensus.go

@@ -47,6 +47,8 @@ var (
 	errInvalidUncleHash = errors.New("non empty uncle hash")
 
 	errUnauthorized = errors.New("unauthorized")
+
+	ErrNoTicket = errors.New("Miner doesn't have ticket")
 )
 
 // SignerFn is a signer callback function to request a hash to be signed by a
@@ -727,7 +729,7 @@ func (dt *DaTong) calcBlockDifficulty(chain consensus.ChainReader, header *types
 		}
 	}
 	if !haveTicket {
-		return nil, nil, 0, nil, fmt.Errorf("Miner doesn't have ticket at block height %v", parent.Number)
+		return nil, nil, 0, nil, ErrNoTicket
 	}
 	ticketsTotalAmount, numberOfticketOwners := parentTickets.NumberOfTicketsAndOwners()
 

miner/worker.go

@@ -788,7 +788,14 @@ func (w *worker) commitNewWork(interrupt *int32, noempty bool, timestamp int64)
 		header.Coinbase = w.coinbase
 	}
 	if err := w.engine.Prepare(w.chain, header); err != nil {
-		log.Error("Failed to prepare header for mining", "err", err)
+		switch err {
+		case datong.ErrNoTicket:
+			if common.DebugMode {
+				log.Info("Miner doesn't have ticket", "number", parent.Number)
+			}
+		default:
+			log.Error("Failed to prepare header for mining", "err", err)
+		}
 		return
 	}
 	// If we are care about TheDAO hard-fork check whether to override the extra-data or not