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:

1. Never raise a policy exception for an omnipotent viewer. I think this is the expected behavior and a reasonable rule.
2. 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.

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