Replaced to return the td and throw a specific error on TD

chain/error.go

@@ -114,3 +114,15 @@ func IsOutOfGasErr(err error) bool {
 
 	return ok
 }
+
+type TDError struct {
+	a, b *big.Int
+}
+
+func (self *TDError) Error() string {
+	return fmt.Sprintf("incoming chain has a lower or equal TD (%v <= %v)", self.a, self.b)
+}
+func IsTDError(e error) bool {
+	_, ok := err.(*TDError)
+	return ok
+}