[FEATURE] Add positive output to the generator

[dblock]

Is your feature request related to a problem?

It's unclear whether the generator worked.

$ npm run merge -- $ROOT_PATH $OUTPUT_PATH

> [email protected] merge
> ts-node merger/merge.ts ../spec ../build/opensearch-openapi.yaml

Path not found: /_plugins/_alerting/destinations
Path not found: /_plugins/_alerting/destinations/email_accounts/_search
Path not found: /_plugins/_alerting/destinations/email_accounts/{emailAccountID}
Path not found: /_plugins/_alerting/destinations/email_groups/_search
Path not found: /_plugins/_alerting/destinations/email_groups/{emailGroupID}
Path not found: /_plugins/_alerting/destinations/{destinationID}
Path not found: /_plugins/_alerting/monitors
Path not found: /_plugins/_alerting/monitors/_execute

What solution would you like?

1. The generator should output some messages such as "Wrote build/opensearch-openapi.yaml successfully."
2. The generator should not output errors as warnings.

Some other ideas:

3. Add --verbose for detailed output.
4. The generator parameters should be named, e.g. npm run merge -- src/... -o output/file.yml --verbose

What alternatives have you considered?

N/A

Do you have any additional context?

I was trying to run the generator and got confused by the output.