Handle HTTP 400 errors thrown when a dependant bug is non-public (issue 55)

Only throws for ! "HTTP status 400", until https://github.com/harthur/bz.js/issues/17 lets us differentiate between private bugs and other bad request failures more easily. 

Also cleaned up some comments & moved the waiting counter decrement until after we've checked for dependant bugs, to avoid any race conditions (however small a window) there were at present.