RF: standardize exceptions for Py 2 / 3

Using long-hand for getting exception error message to preserve Python
2.5 compatibility; go to Python >= 2.6 standard syntax.
Merge pull request #183 from matthew-brett/py3-standard-exceptons

RF: continue py3 update standarizing exceptions

We were using sys.exc_info to work around Python 2.5 / 3 compatibility; use the
new exception syntax, now we depend on Python 2.6