User-defined template error messages

[rikki cattermole]

This is a great example of why I want signatures.

Having a concrete type that gives us a way to verify complex types 
clearly and without using complex CTFE or templates is a must if we want 
to help new users into D in the future.

Most importantly it needs to be baked into the type system enough that 
it allows conversion to a vtable as well as compile time verification of 
the input types implicitly. Based upon my experiments (so the type 
itself can adapt to an implementation).

The other problem is it needs some way to model internal attributes that 
are set externally (e.g. InputRange.Type), my named parameter DIP was 
based upon this requirement.

To go this direction won't be easy. So far my designs result in being 
able to do OOP in them and strangely they seem to be far more like best 
practice OOP than classes ever were. But alas it could just be the fact 
that I designed them talking ;)