#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.

 



> Ah, that's where it's coming from. I was a bit relucant about that proposal tbh. Maybe like so:
> 
> Can't map property "boolean foo" to "int foo". Consider to declare/implement a mapping method "int booleanToInt(boolean b)".
> ?

I suspected as much :smile:. That's why I left the original message intact followed by an advice. I would go to the trouble to name the method otherwise then `map`. You copy and rename it anyway. I did like the idea of aiding the user though, that's why I implemented it. 

So, counter proposal, just:  `Can't map property "boolean foo" to "int foo". Consider to declare/implement a mapping method "int map(boolean b)".`??

BTW. Did you now the word property is actually scenario dependent ? (property|parameter).