Exception if error under test

@mattwire This is fairly simple: if we're under test then there's no point calling statusBounce() so we throw an exception instead which makes more sense when observing test results.

No tests rely on this (yet); it just helps in testing when stuff does go wrong, and won't have any detrimental effect on production.

(I need to post this to see if it's picked up the right commit. Looks a bit wrong at this point.)