Python string exceptions no more allowed in Python 2.6

[Alessio Treglia]

With Python2.6 string exceptions are no longer allowed (they were buggy before too, since they were not guaranteed to work
reliable even in <2.6)

$ python2.5 -c "raise 'eggs'"
-c:1: DeprecationWarning: raising a string exception is deprecated
Traceback (most recent call last):
  File "<string>", line 1, in <module>
eggs

$ python2.6 -c "raise 'eggs'"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
TypeError: exceptions must be old-style classes or derived from BaseException, not str

Please apply the attached patch.