cache/redis: Try to ping redis on cache creation

Otherwise the bridge would continue to run even if the cache is not
available.

cache/redis.go

@@ -22,9 +22,13 @@ func NewRedisCache(redisURL string, d time.Duration) (Cache, error) {
 	}
 
 	rdb := redis.NewClient(ropts)
+	err = rdb.Ping(context.Background()).Err()
+	if err != nil {
+		return nil, err
+	}
+
 	c.client = rdb
 	c.duration = d
-
 	return c, nil
 }