for-each-repo: error on bad --config

for-each-repo: error on bad --config

As noted in 6c62f015520 (for-each-repo: do nothing on empty config,
2021-01-08) this command wants to ignore a non-existing config key,
but let's not conflate that with bad config.

Before this, all these added tests would pass with an exit code of 0.

We could preserve the comment added in 6c62f015520, but now that we're
directly using the documented repo_config_get_value_multi() value it's
just narrating something that should be obvious from the API use, so
let's drop it.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>