folder_branch_ops: handle MDCache Put error
folder_branch_ops: handle MDCache Put error

Issue: #277

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