Log signalR errors as trace when they are network connection aborted

Closes #498

src/NzbDrone.Host/Owin/NlogTextWriter.cs

@@ -64,6 +64,11 @@ private LogLevel GetLogLevel(string value)
                 return LogLevel.Trace;
             }
 
+            if (lower.Contains("signalr") && lower.Contains("the network connection was aborted by the local system"))
+            {
+                return LogLevel.Trace;
+            }
+
             return LogLevel.Error;
         }
     }