Throw CommandException instead of Exception after git fetch failure in repository updates

Summary: Fixes T9966. In this unusual, difficult-to-reach case, we throw Exception (which has no censoring) instead of CommandException (which has censoring). Throw CommandException instead.

Test Plan:

• Hacked up a bunch of stuff in order to hit this: disabled origin validation, origin correction, and pointed repository at a bad domain.
• Verified message is now censored correctly.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9966

Differential Revision: https://secure.phabricator.com/D14745