API: Handle exceptions from ApiBeforeMain hook in a user-friendly manner

API: Handle exceptions from ApiBeforeMain hook in a user-friendly manner

The immediate impetus behind this change is this series of events:

1. CirrusSearch hooks ApiBeforeMain to handle some setup that requires
   the User object.
2. So User is loaded from the session.
3. OAuth checks the headers as part of loading User.
4. OAuth sees that the headers are invalid, and since it was called from
   the API it throws a UsageException, expecting the API to catch it and
   return an appropriate response to the client.
5. But nothing does so, leading to an unhelpful "Internal Error" page
   being returned to the client.

We can do better than that.

Bug: 62312
Change-Id: Ib5735661eec6ebe57eaa69c67b399e703cc90fc4