Consider improving error message when attempting $natural sort on a view

It's currently not possible to use $natural in aggregation to force a "natural sort order". However, if a user attempts a $natural sort when running the find command on a view, they'll get this fairly unnatural-looking error message:

> db.view.find().sort( { $natural: 1 } )

Error: error: {

        "ok" : 0,

        "errmsg" : "FieldPath field names may not start with '$'.",

        "code" : 16410,

        "codeName" : "Location16410"

}

We should consider whether or not to detect this up-front in order to produce a clearer error message for the user.