Update exception to use MemberAccessor

When updating the `SetCollectionValueAsync()` method to use the new `MemberAccessor` (385bfec), I missed the exception message. Oops. (This still compiled, but it should go through `MemberAccessor` for consistency.)

Author: JeremyCaney

OnTopic/Mapping/TopicMappingService.cs

@@ -750,8 +750,8 @@ MappedTopicCache          cache
 
       Contract.Assume(
         targetList,
-        $"The target list type, '{configuration.Property.PropertyType}', could not be properly constructed, as required to " +
-        $"map the '{configuration.Property.Name}' property on the '{target?.GetType().Name}' object."
+        $"The target list type, '{configuration.MemberAccessor.Type}', could not be properly constructed, as required to " +
+        $"map the '{configuration.MemberAccessor.Name}' property on the '{target?.GetType().Name}' object."
       );
 
       /*------------------------------------------------------------------------------------------------------------------------