core: cardigann json: add NoResultsMessage to Search Paths Response block

some json servers in response to a query-not-found return a message string instead of a json object.
so this is a way to avoid a json-object-no-found error and get a graceful no-results-found state.

src/Jackett.Common/Indexers/CardigannIndexer.cs

@@ -1383,6 +1383,8 @@ namespace Jackett.Common.Indexers
                 {
                     if (response.Status != HttpStatusCode.OK)
                         throw new Exception($"Error Parsing Json Response: Status={response.Status} Response={results}");
+                    if (response.Status == HttpStatusCode.OK && SearchPath.Response != null && SearchPath.Response.NoResultsMessage != null && SearchPath.Response.NoResultsMessage.Equals(results))
+                        continue;
                     var parsedJson = JToken.Parse(results);
                     if (parsedJson == null)
                         throw new Exception("Error Parsing Json Response");

src/Jackett.Common/Models/IndexerDefinition.cs

@@ -201,6 +201,7 @@ namespace Jackett.Common.Models
     {
         public string Type { get; set; }
         public string Attribute { get; set; }
+        public string NoResultsMessage { get; set; }
         public bool Multiple { get; set; } = false;
     }
 }