Specify the behavior of "subscribe" when the subscription name already exists

The docs do not specify what happens if the subscription name already exists for a [`subscribe`](https://facebook.github.io/watchman/docs/cmd/subscribe.html) command.

Similarly, behavior is undefined when the subscription name doesn't exist for an [`unsubscribe`](https://facebook.github.io/watchman/docs/cmd/unsubscribe.html) command.