core/rawdb: provide more info on 'gap in the chain' error (ethereum#2…

…5938)

core/rawdb/database.go

@@ -240,8 +240,8 @@ func NewDatabaseWithFreezer(db ethdb.KeyValueStore, ancient string, namespace st
 			if kvhash, _ := db.Get(headerHashKey(frozen)); len(kvhash) == 0 {
 				// Subsequent header after the freezer limit is missing from the database.
 				// Reject startup if the database has a more recent head.
-				if *ReadHeaderNumber(db, ReadHeadHeaderHash(db)) > frozen-1 {
-					return nil, fmt.Errorf("gap (#%d) in the chain between ancients and leveldb", frozen)
+				if ldbNum := *ReadHeaderNumber(db, ReadHeadHeaderHash(db)); ldbNum > frozen-1 {
+					return nil, fmt.Errorf("gap in the chain between ancients (#%d) and leveldb (#%d) ", frozen, ldbNum)
 				}
 				// Database contains only older data than the freezer, this happens if the
 				// state was wiped and reinited from an existing freezer.