json.net has key method?

Just use x["error_msg"]. If the property doesn't exist, it returns null.

JObject implements IDictionary<string, JToken>, so you can use:

IDictionary<string, JToken> dictionary = x;
if (dictionary.ContainsKey("error_msg"))

... or you could use TryGetValue. It implements both methods using explicit interface implementation, so you can't use them without first converting to IDictionary<string, JToken> though.

JObject.ContainsKey(string propertyName) has been made as public method in 11.0.1 release

Documentation - https://www.newtonsoft.com/json/help/html/M_Newtonsoft_Json_Linq_JObject_ContainsKey.htm