fix for issue #539 -- abs() nonsensical in the unsigned domain 

@pitrou, do you see anything problematic about waiting until all the arguments have been converted before checking for Python exceptions?  It seems OK in a quick test (two arguments, first one generates the exception), but I don't know if we could get into trouble if we make several CPython API calls in a row without checking for errors.

(The alternative is to check for an error after each argument conversion, but I don't want to do that unless it is necessary for correctness.)