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(1)  Why change the exception style?

When raising an instance, the style guide (PEP-8, http://
www.python.org/dev/peps/pep-0008/) prefers to construct 
that instance; the older form is left over from String 
exceptions and will be removed in Python 3.

I could understand leaving them as they were, but if you're 
going to change them to make them consistent, why not use 
the current format?

(2)  Why get rid of the debug messages (such as the 
checksum check) entirely?  Guarding them in if self.debug, 
I would understand.

(3)  I wouldn't count on str(e) (where e is any ValueError 
instance) being as meaningful as the (current version's) 
ReadError("empty, unreadable or compressed file")