Properly translate Gitaly NotFound error in poller
Even after a recent fix Update Gitaly error status code (!1253 - merged), we are still getting some NotFound
error in Sentry. See https://new-sentry.gitlab.net/organizations/gitlab/issues/469337/events/3539968da6db43ce96051e733b909fac/?project=11.
This is because there is one more place that is not doing the error translation properly.