error: Swap SMTPConnectionError superclasses

To avoid:

  >>> import rss2email.error
  >>> rss2email.error.SMTPAuthenticationError(server='host', username='bob'))
  Traceback (most recent call last):
    File "<string>", line 1, in <module>
    File "./rss2email/error.py", line 85, in __init__
      server=server, message=message)
    File "./rss2email/error.py", line 67, in __init__
      super(SMTPConnectionError, self).__init__(message=message)
  TypeError: SMTPAuthenticationError does not take keyword arguments

Reported-by: Federico Churca-Torrusio <fchurca@fi.uba.ar>
Signed-off-by: W. Trevor King <wking@tremily.us>