small tweaks to getBridgeStatus.php

Nice work Myata!
My only change is to make the status passable/impassable.  Impassable
means at least one span is down.  Destroyed means more than half are
down.  And 'operateable' is not an English word.  'Passable' conveys the
meaning we intend.  'Functional' would also do, but not quite as well.