R1:fa6cd200e878

Reduce the severity of policy fatals when building the Harbormaster "build status" element

Summary:
See PHI430. Ref T13102. When the "Build Status" element raises a policy exception, we currently fatal the whole page rather than raising a normal policy error.

This is because the policy check happens very late in page construction, long after we've made the decision to show the page instead of a policy error, and gets treated as a rendering error.

In turn, this is because the rendering is event-based…