#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)".

?