Await calls with try/catch

In 451eddc647045bc6309bc0c31ca653a042d19fee we were a little overzealous
in removing async/await calls. Anywhere we do a try/catch around a
method that returns a Task, we need to use await.
Fixed a couple more tests

We don't need to await tests of argument validation. But we do need to
await tests that test exceptions returned from a task.
Merge pull request #437 from octokit/haacked/regression-fix

Fix regression in throwing proper 2FA exception