Return SAST report errors in an `errors` array

What does this MR do and why?

Return SAST report errors in an errors array

When originally opened in February, the situation was such that:

• this endpoint returned a 200 response if parsing failed
• the response was empty when the parsing failed

Having picked this issue up to work on, I discovered that:

• the endpoint is now correctly returning an error response (400)
• the response body now has an error string in a status_reason field

I synced up with front-end because, even with an error response, the widget is still showing a green dot indicating an "all-ok" security scanning situation

We decided that we should:

• moved that status_reason error string into an errors: [] per the issue description
• let front-end investigate why the existing error response is displaying as a passing response in the widget UI

Once we have the errors: [] interface in place between the front-end and back-end, and the error rendering in the widget UI, we can further iterate on the level of detail returned via the errors: [] array

related to: #390200 (closed)

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Changelog: fixed
EE: true

Screenshots or screen recordings

no error scenario

Before	After

error scenario

Before	After

How to set up and validate locally

1. clone this repo into an EE enabled group: https://gitlab.com/gitlab-org/govern/demos/sandbox/minac/static-reports/mr-widget-with-malformed-reports
2. (optional) For testing, you can replace with_reactive_cache to without_reactive_cache here (docs on reactive caching)
3. follow the README to recreate the error scenarios
4. you should see an error: [] array in the response with this change

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.