Add Error Handling Examples for /users/info in Documentation

[RickDeb2004]

Issue Description

The documentation for the /users/info endpoint in the Mailchimp Transactional API (https://mailchimp.com/developer/transactional/api/users/) lacks examples of error responses, making it difficult for developers to implement proper error handling. When calling /users/info with an invalid API key or under rate-limiting conditions, the API returns errors, but the specific response formats (e.g., JSON structure, HTTP status codes) are not documented. For example, an invalid API key might return {"error": "Invalid API key", "code": 401}, but this isn’t specified. This omission leads to trial-and-error debugging and potential integration failures in production.

Expected behavior: The documentation should include a section with sample error responses (e.g., 401 Unauthorized, 429 Too Many Requests) alongside success examples, similar to other well-documented APIs.

Steps to Reproduce

1. Use a Node.js client (e.g., @mailchimp/mailchimp-transactional) or cURL to call the /users/info endpoint with an invalid API key:

   curl -X GET "https://mandrillapp.com/api/1.0/users/info.json" --data "key=INVALID_KEY"