#339 clarification of error messages

This PR addresses the 
1. unclarity in ambiguous error method coming from #65 
2. coding hint suggested in #321. Here is chosen to keep the original message, extended with a more simple coding hint then suggested.

 
> Hum, yes, good point. We also could just make it

Sorry.. I failed to explain myself here.. Its currently already scenario dependent. The current solution already tells you that's a parameter or property.