folder_block_ops: fix missing err check in truncateLocked
folder_block_ops: fix missing err check in truncateLocked

Yikes!

Issue: KBFS-1567

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