swc-installation-test-2.py: Avoid BaseException.message warning

swc-installation-test-2.py: Avoid BaseException.message warning

Dodge the logic which (at least in Python 2.6.1) raises:

  DeprecationWarning: BaseException.message has been deprecated as of Python 2.6

We're setting the .message attribute explicitly, so the deprecation
warning does not apply to us.  The fix was suggested by Brett Cannon:

On Sun Jul 8 02:42:18 CEST 2007, Brett Cannon wrote [1]:
> You can get around this easily enough with a subclass that uses a
> property for message::
>
>   class gerror(Exception):
>     def _get_message(self, message): return self._message
>     def _set_message(self, message): self._message = message
>     message = property(_get_message, _set_message)

[1]: http://mail.python.org/pipermail/python-dev/2007-July/073777.html