Fix crash parsing search result hits

With the latest server version there have been a couple of instances where the C++ SDK will crash when parsing the hits w/in the search response. We should move the .at() -> .optional() and also try/catch the parsing so that we at least return an error instead of crashing.

Examples have been:

• With 7.6 if you disable scoring, the score is not returned in the hit (it was returned in previous versions)
• At one point 7.6 was returning not returning the index field and instead returning index_names. This was a bug that was fixed, but still, the C++ SDK should not crash when parsing.

Example search request:

{"ctl":{"timeout":75000},"explain":false,"query":{"term":"auto"},"score":"none","size":10}

Example 7.2.5 response (just showing a hit):

{

    "id": "e35ac43a::13",

    "index": "test-search-index_15a633800e3a4641_4c1c5584",

    "score": 0.19669274441134066,

    "sort":

    [

        "_score"

    ]

},

Example 7.6.0-2169 response (just showing a hit):

{

    "id": "22f3df03::13",

    "index": "test-search-index_298b1592a35c2c64_4c1c5584",

    "sort":

    [

        "_score"

    ]

},