Prevent crash when build step has been deleted on build plan

Summary: This prevents crashes when looking at builds, where the build steps have been deleted on the build plan since the build was run. Currently the only information that's pulled from the build step is the description (because this was too large to copy to every target).

Test Plan: Tested it locally.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D10361