Ensure errors are sent to stderr

Summary:
This avoid to generate template like:

- DB_USER: A positive integer id is expected.
- DB_PASSWORD: Usage: getcredentials <id> [password|username|title]

Test Plan: getcredentials > aaaa

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1859