Improve error descriptions in CP server responses

[Ayush-Yadav]

Enhances the clarity and observability of error responses returned by the CP when requesting the IP address of an Oyster job. The following improvements are included:

• Validation errors for invalid parameters (e.g., empty job_id or region)
• Explicit error when the instance associated with the job ID is not found
• Clear indication when the instance is still initializing and the IP is not yet available
• Improved internal error reporting for AWS-related failures