#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 suspected as much :smile:

:-)

> So, counter proposal, just...

+1 Seems to be a good middle ground.

> BTW. Did you now the word property is actually scenario dependent ?

Hum, yes, good point. We also could just make it 

> Can't map "boolean foo" to "int foo". Consider to...