Stabilize fatals when a build has a build plan the viewer can't see because of policy restrictions

Summary:
Ref T13526. Currently, if a build plan is restricted, viewers may fatal when trying to view related builds.

The old behavior allowed them to see the build even if they can not see the build plan. This is sort of incoherent, but try to stabilize things before fixing this.

Test Plan:
This is a muddy change.

• Created a build with a build plan that Alice can't see.
• As Alice, viewed the build page (restricted before, restricted after); the buildable page (fatal before, works after).
• Also viewed a revision page (works before and after, but user-reported fatal).

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13526

Differential Revision: https://secure.phabricator.com/D21194