libgit: when repo doesn't exist, give readers a NoSuchRepoError
libgit: when repo doesn't exist, give readers a NoSuchRepoError

Suggested by jzila.

Issue: #1228

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