A more precise return type for the handle function

Although the `handle` function in the `Mail` module returns `Unit` it can also throw exception in the case of an unknown message type.

We should change the return type from `Unit` to something that includes the case of failure, something like:
* [Try](http://www.scala-lang.org/files/archive/nightly/docs/library/index.html#scala.util.Try) in Scala
* [Either](https://hackage.haskell.org/package/base-4.7.0.1/docs/Data-Either.html) in Haskell
* [Choice](http://msdn.microsoft.com/en-us/library/ee353439.aspx) in F# - although in my opinion it's extremely undescriptive

This Pull Request changes the `handle` function in the `Mail` module to return:
```
Either<unit, InvalidOperationException>
```

If we take this direction, we can probably use the introduced `Either` type as the monadic value in #6.
One of the Discrinimated Unions from Scott Wlaschin's [Railway oriented programming](http://fsharpforfunandprofit.com/rop/) talk on NDC must be this one [here](https://github.com/swlaschin/Railway-Oriented-Programming-Example/blob/master/src/FsRopExample/Controllers.fs#L21-59).

FWIW, I just [added a commit](bd87ddb) which simplifies the return value of the failure case to a String.