我在优胜美地，我安装了Brew。我正在尝试安装redis，我收到以下错误。我该如何解决？
$ brew install redis
==> Downloading https://downloads.sf.net/project/machomebrew/Bottles/redis-2.8.19.yosemite.bottle.tar.gz
Already downloaded: /Library/Caches/Homebrew/redis-2.8.19.yosemite.bottle.tar.gz
==> Pouring redis-2.8.19.yosemite.bottle.tar.gz
Error: File exists - /usr/local/etc
Warning: Bottle installation failed: building from source.
==> Downloading http://download.redis.io/releases/redis-2.8.19.tar.gz
Already downloaded: /Library/Caches/Homebrew/redis-2.8.19.tar.gz
==> make -C /private/tmp/redis-XuZC7r/redis-2.8.19/src CC=clang
Error: File exists - /usr/local/etc