folder_block_manager: better style for the error checks
folder_block_manager: better style for the error checks

Suggested by jzila.

Issue: #1593

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