When repeat registration is attempted, try printing the key

[Eyal Rozenberg]

When we attempt to register a second class with the same key in the same factory, an exception is thrown. It would be useful if we were told which class failed to register. Unfortunately, at that point in the code, all we have is the key. Can we print that? ... not necessarily. Certainly can't be assumed. But - we can give it our best shot using some TMP - in case ``some_ostream << key` can work.