This commit tries to settle things with an old PR-4237 which had relaxed
the case where the SetConfig returned an `Unavailable` code error.
Under this circumnstance the PR allowed the upFunc to just emit a warning and
progress further with the login gRPC. Since the login call is consuming
the --management-url coming from the `up` command, it might be possible
to abuse the "Unavailable" code to inject a management URL that is different
from the configured one even though the --disable-update-settings is set
to true (?)