Add function to notify user of success/failure of operation taking in consideration posible unmet preconditions

[Ambro17]

Currently there's no single way to always send a message to a user. One can use the user id as the channel to chat.postMessage but it's not recommended by slack as it will be deprecated.

We should offer an abstraction over this.

It should be able to notify a user in the same chat (private or public channel, or direct message) that the command was invoked.
If certain precondition is not met, show a proper explanation and optionally send a message through slack bot using the user id as channel.