Fixing a bug I'd introduced into the Tracer system which required
core::init to complete in order for Tracers to get written to. That
is, the Tracers asserted that their visibility had been computed
before their output could be flushed. If, however, core::init hasn't
yet been called, or if core::init threw an exception, then the
visibility would not have been calculated. Now the assertion is that
either the visibility has been calculated or core::init has not
yet been called. This should fix the failing app_exception_handling
integration tests for the release_debug mode build.