consensus/clique: make internal error private (#21132)

Co-authored-by: linjing <linjingjing@baidu.com>

consensus/clique/clique.go

@@ -121,9 +121,9 @@ var (
 	// turn of the signer.
 	errWrongDifficulty = errors.New("wrong difficulty")
 
-	// ErrInvalidTimestamp is returned if the timestamp of a block is lower than
+	// errInvalidTimestamp is returned if the timestamp of a block is lower than
 	// the previous block's timestamp + the minimum block period.
-	ErrInvalidTimestamp = errors.New("invalid timestamp")
+	errInvalidTimestamp = errors.New("invalid timestamp")
 
 	// errInvalidVotingChain is returned if an authorization list is attempted to
 	// be modified via out-of-range or non-contiguous headers.
@@ -322,7 +322,7 @@ func (c *Clique) verifyCascadingFields(chain consensus.ChainReader, header *type
 		return consensus.ErrUnknownAncestor
 	}
 	if parent.Time+c.config.Period > header.Time {
-		return ErrInvalidTimestamp
+		return errInvalidTimestamp
 	}
 	// Retrieve the snapshot needed to verify this header and cache it
 	snap, err := c.snapshot(chain, number-1, header.ParentHash, parents)