[Reporting] For internal APIs, do not use 503/500 codes in the Report Download API

[tsullivan]

Refer to the TODO comment in the Reporting code here.

The "download report" endpoint is built to return a custom HTTP status code of 503 or 500 in cases where the report job has not successfully completed.

• 503: Report job is processing but not yet complete
• 500: Report job has failed and execution retries have exhausted.

This functionality is intended only for the "public" download report API, where it's instrumental in helping Watcher and custom scripts automate reports.

Requirements:
For the "internal" download report API, we need a different semantic mechanism to tell the client that the report is not ready for download, or can't be downloaded. The solution should use a response payload body and the HTTP response code should always be 200 to indicate an OK response..

This solution will alleviate error codes that come up in monitoring of serverless projects.

[elasticmachine]

Pinging @elastic/appex-sharedux (Team:SharedUX)