Fix GetAliasResponse bug

[l-trotta]

GetAliasesResponse is a special case because it can return both an exception and the actual response, in case 2 aliases, 1 present and 1 missing, are added to GetAliasesRequest:

{
  "error": "alias [test2] missing",
  "status": 404,
  "example-index": {
    "aliases": {
      "test": {}
    }
  }
}

The error deserializer doesn't recognize example-index (or any other custom field), and so a TransportException is thrown. This PR adds a default case to the error response deserializer so that any additional field is tentatively deserialized into the metadata map of ErrorCause.
So the json above would still the client throw an ElasticsearchException, since there's a 404 error, and the rest of the information would be found in metadata like so:

try{
    client.indices().getAlias(a -> a.name("test","test2"));
}
catch (ElasticsearchException e){
    Map<String, JsonData> metadata = e.error().metadata();
    JsonData index = metadata.get("example-index");
    Map aliases = index.to(Map.class);
    assertEquals("test", aliases.keySet().iterator().next());
}

Fixes #1033, fixes #900