early draft of exceptions PDD

[Allison Randal]

In: docs/pdds/clip/pddXX_exceptions.pod

As with the I/O PDD, this isn't a final form, it's just a draft to  
seed discussion. What's missing? What's inaccurate? What's accurate  
for the current state of Parrot, but is something you always intended  
to write out later? What thoughts have you had on how exceptions  
should work? All comments, suggestions, and contributions cheerfully  
welcomed.

Allison