folder_block_ops: check BOTH err listeners for a context cause
folder_block_ops: check BOTH err listeners for a context cause

There are two places while writes are blocked that can receive an
error from a failed Sync.  This factors out the error-checking code so
it can be used in both places.

Issue: KBFS-2164

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