R1:c207964036ce

Never raise policy exceptions for the omnipotent viewer

Summary:
Fixes T4109. If a revision has a bad `repositoryPHID` (for example, because the repository was deleted), `DifferentialRevisionQuery` calls `didRejectResult()` on it, which raises a policy exception, even if the viewer is omnipotent. This aborts the `MessageParser`, because it does not expect policy exceptions to be raised for an omnipotent viewer.

Fix this in two ways:

# Never raise a policy exception for an omnipotent viewer. I think…