folder_block_ops: clean up blocks based on policy of first failure
folder_block_ops: clean up blocks based on policy of first failure

Otherwise a block that carries over from one failed sync to another
could get incorrectly deleted if the first failed sync actually
succeeded on the master branch.

Issue: KBFS-1569

keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/kbfs,keybase/kbfs,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client