Exit with 79 on a two factor problem; document exit codes

As discussed in #171, let's use a special exit code to denote a problem with two factor authentication.

Also as discussed in #171, let's start a document describing the current state of exit codes used by hk.

``` bash
$ hk env -a paranoid
error: A second authentication factor or pre-authorization is required for this request. Your account has either two-factor or a Yubikey registered. Authorize with `hk authorize`.

$ echo $?
79
```

/cc @dpiddy @dickeyxxx