Mark fallback directory mirrors as "too busy" when they return
a 503 response. Previously, the code just marked authorities as busy.
Unless clients set their own fallback directories, they will never see
this bug. (There are no default fallbacks yet.)
Fixes bug 17572; bugfix on 5c51b3f1f0 released in 0.2.4.7-alpha.
Patch by "teor".
