[Tutor] function signatures for callbacks

[Don Taylor]

Danny:

Thanks for this, I think that I can use this idea.

When I first read your response I did not see how it helped me, but now 
I realise that it allows me to add some context to the exception message.

I don't suppose that I can change the traceback to point at the 
definition of f2 instead of shout() but I can give a better hint to the 
user at why this went wrong.

Don.