Improve error handling in search endpoint

[Syndesi]

Error messages thrown during the search request handling should be somewhat passed on, e.g.:

curl \
  -k \
  -X POST \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer secret-token:PIPeJGUt7c00ENn8a5uDlc" \
  -d '{"debug": true, "steps": [{"type": "elasticsearch-query-dsl-mixin", "query": {"query": {"match": {"description": {"query": "lily"}}}}}]}' \
  https://reference-dataset.ember-nexus.dev/search

Results in this error:

{
  "type": "http://reference-dataset.ember-nexus.dev/error/500/internal-server-error",
  "title": "Internal server error",
  "status": 500,
  "detail": "Internal server error, see log."
}

Which is not useful at all.

The corresponding error inside the server's log file is:

Exception of type Elastic\Elasticsearch\Exception\ClientResponseException got thrown during search step 1 (type
elasticsearch-query-dsl-mixin): 400 Bad Request: {"error":{"root_cause":[{"type":"parsing_exception","reason":"unknown
query [query]","line":1,"col":73}],"type":"x_content_parse_exception","reason":"[1:73] [bool] failed to parse field
[must]","caused_by":{"type":"parsing_exception","reason":"unknown query [query]","line":1,"col":73,"caused_by":{"type":
"named_object_not_found_exception","reason":"[1:73] unknown field [query]"}}},"status":400}

---

Acceptance criteria:

• Add integration tests which query the API with known bad queries, and compare the returned exception text against stored helpful ones.
• Make sure that common errors, e.g. bad Elasticsearch queries, are correctly forwarded to the user.