reassess minimum-dir-info when a bridge fails

When we try to fetch a bridge descriptor and we fail, we mark
the guard as failed, but we never scheduled a re-compute for
router_have_minimum_dir_info().

So if we had already decided we needed to wait for this new descriptor,
we would just wait forever -- even if, counterintuitively, *losing* the
bridge is just what we need to *resume* using the network, if we had it
in state GUARD_REACHABLE_MAYBE and we were stalling to learn this outcome.

See bug 40396 for more details.