Added missing contracts in Directory class.

[fedotovalex]

This fixes #249.

Adds contracts for:

• Directory.GetFileSystemEntries(string, string, SearchOption)
• Directory.GetFiles(string, string, SearchOption)
• Directory.GetDirectories(string, string, SearchOption)

[SergeyTeplyakov]

Is this message is correct? It seems slightly misleading to me because it doesn't tell what the error type is...

[fedotovalex]

Well, that's what MSDN says, although honestly I just copied and pasted this part without much thinking. Looking closer, I'm not sure if there is much value in adding EnsuresOnThrow in this form.

[SergeyTeplyakov]

Yeah.. This stuff looked weird for me as well, but then I've searched thought this repo and found that this is a canonical way to express possible exceptions.

My point was more about the message that is absolutely unclear, but because the official documentation is saying that, I'm not sure what we can do. I would prefer to have more clear message but have no idea what this message should looks like.

[SergeyTeplyakov]

Everything is looks good for me...

[yaakov-h]

Oh thanks! 😄 I was going to attempt this myself, but got stuck fighting a Git-vs-Line-Endings war.