Make `arc shell-complete` more robust.
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

dguo-coursera/arcanist,fivestars/arcanist,eSpark/arcanist,kevinseelbach/arcanist,chundongwang/arcanist,dguo-coursera/arcanist,coursera/arcanist,folsom-labs/arcanist,haskell-infra/arcanist,PageUpPeopleOrg/arcanist-patches,folsom-labs/arcanist,KieranHayes/arcanist,zhihu/arcanist,aik099/arcanist,wharris/arcanist,dguo-coursera/arcanist,aik099/arcanist,hach-que/arcanist,Khan/arcanist,wxstars/arcanist,haskell-infra/arcanist,phacility/arcanist,r4nt/arcanist,Symplicity/arcanist,codelabs-ch/arcanist,zhihu/arcanist,eSpark/arcanist,codelabs-ch/arcanist,sneakpeeq/arcanist,folsom-labs/arcanist,jra3/arcanist,wikimedia/phabricator-arcanist,calblueprint/arcanist,Symplicity/arcanist,KieranHayes/arcanist,coursera/arcanist,sneakpeeq/arcanist,wharris/arcanist,sneakpeeq/arcanist,memsql/arcanist,codelabs-ch/arcanist,KieranHayes/arcanist,fivestars/arcanist,codelabs-ch/arcanist,alexmv/arcanist,wikimedia/phabricator-arcanist,jra3/arcanist,chundongwang/arcanist,memsql/arcanist,wikimedia/phabricator-arcanist,haskell-infra/arcanist,Khan/arcanist,haskell-infra/arcanist,codelabs-ch/arcanist,memsql/arcanist,memsql/arcanist,wxstars/arcanist,hach-que/arcanist,hach-que/arcanist,eSpark/arcanist,coursera/arcanist,uber/arcanist,eSpark/arcanist,calblueprint/arcanist,zhihu/arcanist,sneakpeeq/arcanist,uber/arcanist,hach-que/arcanist,coursera/arcanist,phacility/arcanist,PageUpPeopleOrg/arcanist-patches,dguo-coursera/arcanist,chundongwang/arcanist,r4nt/arcanist,kevinseelbach/arcanist,fivestars/arcanist,fivestars/arcanist,jra3/arcanist,aik099/arcanist,wikimedia/phabricator-arcanist,Khan/arcanist,phacility/arcanist,wxstars/arcanist,wharris/arcanist,jra3/arcanist,Symplicity/arcanist,alexmv/arcanist,PageUpPeopleOrg/arcanist-patches,Symplicity/arcanist,aik099/arcanist,phacility/arcanist,alexmv/arcanist,wxstars/arcanist,KieranHayes/arcanist,kevinseelbach/arcanist,wharris/arcanist,r4nt/arcanist,chundongwang/arcanist,alexmv/arcanist,calblueprint/arcanist,zhihu/arcanist,calblueprint/arcanist,uber/arcanist,PageUpPeopleOrg/arcanist-patches,kevinseelbach/arcanist,uber/arcanist,r4nt/arcanist,folsom-labs/arcanist,phacility/arcanist,Khan/arcanist