Introduce event-message into Error tracking api

We currently support the Sentry data types in the following table. We should add support for the message interface for events. Currently GOB can parse successfully an event containing the Message interface however we throw 400 bad request for any event that is not an exception (check out the code).

GOB should support events containing the message interface. With this issue we can introduce the support needed by storing the events instead of discarding them with 400.