Reads an approval request object with a given id.

def ReadApprovalRequest(self, requestor_username, approval_id):
    """Reads an approval request object with a given id."""
    try:
      return self.approvals_by_username[requestor_username][approval_id]
    except KeyError:
      raise db.UnknownApprovalRequestError("Can't find approval with id: %s" %
                                           approval_id)