Surface per-search multi_search errors #37

Typesense can return HTTP 200 from /multi_search while an individual result is an error:

{"code": 400, "error": "Could not find a filter field named ..."}

Index::searchUsingApi() assumes that result always contains hits and calls count($searchResults['hits']). The useful Typesense error is therefore replaced by Undefined array key \"hits\".

Validate the individual result before reading hits. When it contains code/error, throw a TypesenseClientError carrying the original message and status. Add a regression test with a mocked multi-search response so it does not require a live Typesense server.

Found via transformstudios/zakat.org#2309. Parent review: transformstudios/zakat.org#2374.