BaseException.message is deprecated since Python 2.6

PEP 352 deprecated the message attribute of the BaseException class.
Using the message attribute will result in warnings like this:

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

Using unicode(exc) is the suggested replacement.

Change-Id: Ie322aaa69c025ae5bf39d00f7e3487fe2527545b