Handle unhappy paths with generic discriminated unions

- [Lecturer] So now we need to go ahead…and worry about what happens…if we get various invalid forms of inputs,…and because that's going to happen particularly…in an open environment like Twitter,…let's kick off, let's just cheat a little bit…and copy some of the boilerplate from our tests.…And, we're going to say, "given some invalid text."…I'm going to take out those.…I'm going to copy the test fixture stuff,…and I suppose the most trivial form…of invalid input is just an empty string,…leaving aside nulls for now.…

I got to go back to my silly test now…of comparing false to false.…And, then the real question becomes…what in fact should we do there?…What should the parser return?…Should we say the parser returns an exception,…or the parser returns empty stuff?…Neither of those seem particularly satisfactory.…So, we're going to really explicitly model…what could potentially happen there,…and I'm going to just put that…modeling in the test code for now.…I'm going to declare a new type,…and I'm going to call the type…