test_build: exit rather than hanging on event handler exception
    
    Event handlers decorated with @celery_app_task don't raise an exception -
    they just log the exception, leaving the Moksha hub running. This meant
    that any failures in test_build would result in the test suite hanging
    rather than failing usefully.
    
    We solve this by adding a new class EventTrap which acts as a context
    manager. Any exceptions that occur in event handlers are set on the
    current EventTrap, and the test_build tests re-raise the exception.