Fail more softly on funky arcanist project / symbol data

Summary: Fixes T8240, probably?

Test Plan: This is hard to test locally post-migration -- @btrahan, does it fix things for you?

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: btrahan, epriestley

Maniphest Tasks: T8240

Differential Revision: https://secure.phabricator.com/D12903