#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.

 
I'm no huge fan of the "no property mapper". How about this:

> Can't map property "boolean foo" to "int foo", no suitable mapping method found.