Handle non-zero exit code

[joshbetz]

VIP CLI should send a non-zero exit code with the WP-CLI command exits with a non-zero code. We can test this with the following command.

WP_CLI::add_command( 'test exit-code', function() {
    WP_CLI::error( 'An error has occurred.' );
} );

Currently, VIP CLI always exits this command with an exit code of 0, but it should be 1.

This will likely involve work on the API and runner to pass the exit code along.

Related: #361

[github-actions]

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