correctly refer to exception

this is left-over from 87d354e1a013806ef37f11e22f015e5cc43e533d

Addressing:
Traceback (most recent call last):
  File "/usr/bin/osc", line 26, in <module>
    r = babysitter.run(osccli)
  File "/usr/lib/python2.7/site-packages/osc/babysitter.py", line 60, in run
    return prg.main()
  File "/usr/lib/python2.7/site-packages/osc/cmdln.py", line 335, in main
    self.postoptparse()
  File "/usr/lib/python2.7/site-packages/osc/commandline.py", line 136, in postoptparse
    override_verbose = self.options.verbose)
  File "/usr/lib/python2.7/site-packages/osc/conf.py", line 873, in get_config
    add_section(conffile, url, user, passwordx)
  File "/usr/lib/python2.7/site-packages/osc/conf.py", line 712, in add_section
    except OscConfigParser.ConfigParser.DuplicateSectionError:
AttributeError: class OscConfigParser has no attribute 'ConfigParser'