Allow search to continue when sort field is missing from type mapping

[bryangreen]

I'd like the search to ignore the sort key when it is missing from a type's mapping.

This could be an option for the sort key such as "ignore_unmapped": true

For instance:

{
    "from": 0,
    "size": "15",
    "sort": {
        "title": {
            "order": "asc",
            "ignore_unmapped": true
        }
    }
}

I have this error throwing on some indexes and sometimes throws the search. This would be a great feature!

Updated references to "ignore_unmapped"