How to display error message from indexer in CLI

[JanPetr]

I have an indexer, which needs to be configured first - requires API credentials.

When I run in a reindex command in CLI: $ bin/magento indexer:reindex my_reindexer_name and the API credentials are not set, I want to display an error / warning message telling the user to configure his credentials. Currently I'm doing it like this:

if ($wrongCredentials) {
    $errorMessage = 'You need to configure your credentials';

    if (php_sapi_name() === 'cli') {
        throw new \Exception($errorMessage);
    }

    $this->messageManager->addErrorMessage($errorMessage);

    return;
}

The issue with this approach is that when the Magento is installed from CLI (via $ bin/magento setup:install ...), the Magento tries to reindex the data, the Exception is thrown (of course, API credentials are not set yet) - so the installation crashes. But I don't want to break the install, simply displaying the message is fine.

When I replace throw new \Exception(...); by simple echo $errorMessage (which works), Magento Marketplace rejects the extension from technical review (bad echo placement).

When I remove the CLI condition completely, $messageManager won't display the error message in CLI.

So the question is - how correctly display the error message in CLI without breaking the installation? Thanks!

[JanPetr]

Addition:

In the indexer I don't have access to OutputInterface. The "reindex" command doesn't forward it to the indexers. And OutputInterface cannot be instantiated via DI either.

[veloraven]

@JanPetr thank you for your feedback.
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