partis: handle internal server error

when partis is offline  we currently get this obscure error:
Exception (partis): Cannot perform runtime binding on a null reference
now we get:
Exception (partis): Partis is offline, returning an Internal server error

src/Jackett.Common/Indexers/Partis.cs

@@ -175,7 +175,8 @@ namespace Jackett.Common.Indexers
 
             // get results and follow redirect
             results = await RequestWithCookiesAsync(searchUrl, referer: SearchUrl, headers: header);
-
+            if (results.ContentString.Contains("Internal server error"))
+                throw new Exception("Partis is offline, returning an Internal server error");
             // parse results
             try
             {