template constraint diagnostics

[Trass3r via Digitalmars-d]

> I don't think it would be too difficult. You are analyzing an 
> expression that evaluates to false, and it wouldn't take much 
> to dig down to find out which subexpressions cause the false to 
> occur.

I think it's not that straightforward in dmd as it simply 
delegates the constraint to the interpreter and uses the 
resulting bool.