fix: handle errors in multisearch response

[cdmbr]

bug: multisearch response with a non 2XX status is handled as successful response (MultiSearchResponse(Nil))

sample request:

GET /_msearch
{}
{"query" : {"exists" : { "field": ""}}}
{}
{"query" : {"exists" : { "field": "ok"}}}

{
  "error": {
    "root_cause": [
      {
        "type": "illegal_argument_exception",
        "reason": "field name is null or empty"
      }
    ],
    "type": "illegal_argument_exception",
    "reason": "field name is null or empty"
  },
  "status": 400
}

[cdmbr]

fyi @sksamuel

[sksamuel]

lgtm