[Wilds] Slightly simplify fatal handling during "arc" setup

Summary:
Ref T13098. We currently handle fatals by printing a message and returning an error code, since this was the most direct adaptation from the old arcanist.php script.

Instead, throw an exception and then handle it above, since we can reasonably do that in one place now.

Test Plan: Ran arc.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13098

Differential Revision: https://secure.phabricator.com/D19693