Throw an exception if field mapping is invalid

Currently we silently ignore invalid field mappings, eg:

```
PUT test1
{
  "mappings": {
    "searchText": {
      "properties": {
        "foo": {
          "type": "string",
          "anlayzer": "snowball",
          "foo": "bar"
        }
      }
    }
  }
}
```

We should throw an exception instead.

Related to #8870
