Add error convenience accessor

[3lvis]

Useful for when you don't care about the result but more if there's an error or not.

Before:

networking.get("/somepath") { result in 
    switch result {
    case .success:
        completion(nil)
    case .failure(let response):
        completion(response.error)
    }
}

Now:

networking.get("/somepath") { result in 
    completion(result.error) // NSError?
}