Don't use deprecated BaseException.message

When running the unit tests, I'd get a warning message:

 /opt/stack/keystone/tests/test_exception.py:151: DeprecationWarning: BaseException.message has been deprecated as of Python 2.6

We shouldn't be using deprecated functions.

This change makes it so the test doesn't use the deprecated member.

Also, test_unicode_message wasn't a test for security error
formatting so moved it to ExceptionTestCase.

Change-Id: I088bf94433baf0f00d3ff8ca63e5f74e9d300cd0
Merge "Don't use deprecated BaseException.message"