In the example you give it seems to me that the message for the new style class is more accurate and useful than the message for the old style class.  Looking at your mercurial example, it looks like the problem is the same (len returning a long), and again it seems to me that the new message is more accurate than the old, and that neither one is more confusing than the other in the given context(s).  Or, if anything, that the OverflowError is less confusing as it has more connection with the problem (committing *large* amounts of data).

I don't see this as a bug in any sense, myself.  What would consider to be a "specific" error message?  Perhaps this is a request for an enhancement, instead.