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.
After doing some search, I think, there is no benefit in creating an Exception only to return it; other than the Message property, which is a String, all the other properties (e.g. StackTrace) are null.

Perhaps, it's better to simply return a String for the value of the failure case. Idiomatically, the value of the failure case is *"..[often fixed to be a string](https://www.fpcomplete.com/school/starting-with-haskell/basics-of-haskell/10_Error_Handling#either-may-be-better-than-maybe)"*.

Then, as we go along, we could probably consider creating a dedicated type to hold the failure messages.