Log failure cause

We encountered an issue where we saw the failure message (`memcached:11211 failed (count: 0)`) appearing in our Resque logs. The reason turned out to be that we were sharing a client instance between parent process and subprocess without closing the connection before forking. To help diagnose the issue, I added additional logging to the `failure!` method. I thought this could be a useful addition to the library. Let me know what you think.