kbfsmd: improve `ServerErrorLockConflict` error message
kbfsmd: improve `ServerErrorLockConflict` error message

It's possible that the user actually sees this message in some rare
cases.

Issue: KBFS-3407

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