Added errors

core/error.go

 package core
 
 import (
+	"errors"
 	"fmt"
 	"math/big"
 )
 
+var (
+	BlockNumberErr = errors.New("block number invalid")
+	BlockFutureErr = errors.New("block time is in the future")
+)
+
 // Parent error. In case a parent is unknown this error will be thrown
 // by the block manager
 type ParentErr struct {