[Csharp] Exception and status code support

[Sandor]

• Use the appropriate base exception based on the status code. geeklearningio/gl-dotnet-restkit#5
• Parse the content of the request if type information is provided

[Sandor]

In addition to error response status, template will accept an option useOkSchema to indicate that all responses should provide the same schema as OK.

 "templateOptions": {
                "clientName": "myClient",
                "projectGuid": "someGuid",
                "rootNamespace": "My.Client.Namespace",
                "operation": {
                    "onError": {
                        "useOkSchema": true
                    }
                }
            }