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: Ibf3c56e4baa6ad83e2b95a948787e9d02cf074d4
Merge "BaseException.message is deprecated since Python 2.6"