Stop throwing errors if no results (yet)

[jurajmajerik]

Bug Description

Currently, if the experiment results could not be calculated due to missing events, we throw the following error with the response code 400 BAD REQUEST:

"[ErrorDetail(string='{"no-events": true, "no-flag-info": true, "no-control-variant": true, "no-test-variant": true}', code='no-results')]"

This is a relic of the old calculation API. In the absence of experiment results, we shouldn’t throw an error, as nothing is necessarily broken - there are simply missing events. We should return a success response with a descriptive status code and a status message.

This logic lives in experiment_trends_query_runner.py and experiments_funnels_query_runner.py

Additional context

Debug info

- [ ] PostHog Cloud, Debug information: [please copy/paste from https://us.posthog.com/settings/project-details#variables or https://eu.posthog.com/settings/project-details#variables]
- [ ] PostHog Hobby self-hosted with `docker compose`, version/commit: [please provide]
- [ ] PostHog self-hosted with Kubernetes (deprecated, see [`Sunsetting Kubernetes support`](https://posthog.com/blog/sunsetting-helm-support-posthog)), version/commit: [please provide]