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