Handle potential null return codes in GetReturnCode()

The `GetReturnCode()` method assumes that the operation it is applied to will return a `@ReturnCode` (or a parameter corresponding to the `sqlParameter` parameter). If the operation doesn't return one, an unintuitive and unexpected exception is thrown.

This is mitigated in this commit by instead falling back to a generic error code of `-1`. This addresses, for instance, a bug where the `GetTopicId()` function wasn't returning a value if the `@TopicKey` parameter couldn't be found. That bug has since been fixed (d517971)—but we should be setup to handle that potentiality regardless. This also provides flexibility for databases that haven't updated `GetTopicId()` yet.

Author: JeremyCaney

OnTopic.Data.Sql/SqlCommandExtensions.cs

@@ -33,7 +33,10 @@ internal static int GetReturnCode(this SqlCommand command, string sqlParameter =
         $"The call to the {command.CommandText} stored procedure did not return the expected 'ReturnCode' parameter."
       );
       var returnCode = command.Parameters[$"@{sqlParameter}"].Value.ToString();
-      return Int32.Parse(returnCode, CultureInfo.InvariantCulture);
+      if (Int32.TryParse(returnCode, NumberStyles.Integer, CultureInfo.InvariantCulture, out var returnValue)) {
+        return returnValue;
+      }
+      return -1;
     }
 
     /*==========================================================================================================================