Better error message when setting values on directories

Without this commit etcd returns the following error message when
setting values on directories:

    {
      "errorCode":102,
      "message":"Not a file",
      "cause":"/postgres",
      "index":2
    }

While the above error message is accurate it's not very descriptive.
This commit adds a new error code/message which better describes why the
write operation failed. etcd now returns the following:

    {
      "errorCode":109,
      "message":"Cannot set value on directory",
      "cause":"/postgres",
      "index":2
    }
Merge pull request #504 from kelseyhightower/master

Better error message when setting values on directories