storage: always return ErrKeyNotFound when key is not found

It was true for MemoryStore and BoltDB, but not for LevelDB and Redis.

pkg/core/storage/leveldb_store.go

@@ -41,7 +41,11 @@ func (s *LevelDBStore) Put(key, value []byte) error {
 
 // Get implements the Store interface.
 func (s *LevelDBStore) Get(key []byte) ([]byte, error) {
-	return s.db.Get(key, nil)
+	value, err := s.db.Get(key, nil)
+	if err == leveldb.ErrNotFound {
+		err = ErrKeyNotFound
+	}
+	return value, err
 }
 
 // PutBatch implements the Store interface.

pkg/core/storage/redis_store.go

@@ -40,6 +40,9 @@ func (s *RedisStore) Batch() Batch {
 func (s *RedisStore) Get(k []byte) ([]byte, error) {
 	val, err := s.client.Get(string(k)).Result()
 	if err != nil {
+		if err == redis.Nil {
+			err = ErrKeyNotFound
+		}
 		return nil, err
 	}
 	return []byte(val), nil