opDispatch, duck typing, and error messages

[Andrej Mitrovic]

> This is a more general problem of runtime errors, not just of to!(). Maybe
> exceptions nature should be changed a little so they store __FILE__ and
> __LINE__ on default (exceptions without this information are kept on
> request, for optimization purposes).

Vote up for that. At least it should do that in -debug mode. It's
completely useless getting an error message with a file and line
number for an internal Phobos function, like some funcImpl() private
function, when the fault is a runtime argument.