#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.

 
Agreed.. I did not tackle that one yet. I also think it could be be less verbose. BTW, I collected all error messages I could find in  #339 . I only tackled 2 of them..