folder_branch_ops: clean up blocks on failure in finalizeGCOp
folder_branch_ops: clean up blocks on failure in finalizeGCOp

Suggested by @akalin-keybase

Issue: #33

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