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