Add ability to send message

https://www.pivotaltracker.com/story/show/78628040
What I'm getting at here is that we cannot say this is exclusively an error in the domain logic (as an aside: do we even have a domain?) that should be resolved immediately. This is only the case if our validation doesn't match the prescribed rules in the messagebird docs. There are however also errors possible with an http status code in the 4xx range that have something to do with the account (error code 25, not enough balance) which is most certainly not an exception in the domainlogic.

If an exception is thrown, I would indeed vote for a `RuntimeException`, or split it out in more finegrained detection.