Make `arc shell-complete` more robust.

Summary: Fixes T5122. Add some basic error handling to the `shell-complete` workflow.

Test Plan: `arc shell-complete` no longer throws errors.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T5122

Differential Revision: https://secure.phabricator.com/D9268