core: return error if repair block failed (#18126)

* core: return error if repair block failed

* make error a bit shorter

core/blockchain.go

@@ -445,7 +445,11 @@ func (bc *BlockChain) repair(head **types.Block) error {
 			return nil
 		}
 		// Otherwise rewind one block and recheck state availability there
-		(*head) = bc.GetBlock((*head).ParentHash(), (*head).NumberU64()-1)
+		block := bc.GetBlock((*head).ParentHash(), (*head).NumberU64()-1)
+		if block == nil {
+			return fmt.Errorf("missing block %d [%x]", (*head).NumberU64()-1, (*head).ParentHash())
+		}
+		(*head) = block
 	}
 }