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 this is the expected behavior and a reasonable rule.
  # In this case, load the revision for an omnipotent viewer.

This feels a little gross, but it's the only place where we do this in the codebase right now. We can clean this up later on once it's more clear what the circumstances of checks like these are.

Test Plan: Set a revision to have an invalid `repositoryPHID`, ran message parser on it, got a clean parse.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4109

Differential Revision: https://secure.phabricator.com/D7603