Improve error message when uploading invalid run payload for a datastore

[lampajr]

Feature idea.

Currently Tests support only once single backend datastore, either Postgres (the default), Elasticsearch or Collector API (see https://horreum.hyperfoil.io/docs/integrations/ for more details). Therefore, if a user tries to upload a generic JSON as payload on a Test backed by Elasticsearch it will simply get 400 with no meaningful message about the error itself (i.e., you need to upload a valid ES query)

Describe the solution you'd like

Improve the error message when the specific Datastore cannot parse the payload request accordingly, and return back a meaningful message

Additional information
n/a