Fail more gracefully when rendering transcripts if handle is missing

Summary: Fixes T4628. I can only partially reproduce the root cause here, but if transcript display rules aren't quite right we should just degrade here rather than fatalling. Transcripts are a messy business by any measure.

Test Plan: Sort-of-reproing transcript renders OK now.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4628

Differential Revision: https://secure.phabricator.com/D8554