folder_block_manager: error wording update
folder_block_manager: error wording update

Suggested by @akalin-keybase.

Issue: #327

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