R1:89d88dafcc41

Fix a Differential exception in invalid/missing fields

Summary: Ref T11114. Missed this while converting.

Test Plan: Tried to create a revision with no test plan. Before: fatal; after: helpful message.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11114

Differential Revision: https://secure.phabricator.com/D17061