kbfsmd: return version error if incorrectly asked to make iteam MD
kbfsmd: return version error if incorrectly asked to make iteam MD

If the mdserver turns on support for iteams, but we are running
without that support turned on yet (i.e., in prod mode), we should
return a NewMetadataVersionError instead of a generic error, so that
the user will get a popup about needing to upgrade instead of just a
plain EIO.

Issue: KBFS-2621

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