Add a callback for gcs table lookup failures. (#1702)

* Add callback to gcs client for table lookup failures.

* update plasma_manager reflecting changes to gcs callback.

src/common/redis_module/ray_redis_module.cc

@@ -524,6 +524,9 @@ int TableLookup_RedisCommand(RedisModuleCtx *ctx,
   RedisModuleString *id = argv[2];
 
   RedisModuleKey *key = OpenPrefixedKey(ctx, "T:", id, REDISMODULE_READ);
+  if (key == nullptr) {
+    return RedisModule_ReplyWithNull(ctx);
+  }
   size_t len = 0;
   const char *buf = RedisModule_StringDMA(key, &len, REDISMODULE_READ);