Fix DeprecationWarning when printing exception

On Python 2.6, this results in an error like:
/usr/lib/python2.6/site-packages/cinderclient/shell.py:524:
DeprecationWarning: BaseException.message has been deprecated as of
Python 2.6
  message = e.message

Should use the method from novaclient commit 8c4e145b92, which
works well with python2 and python3.

Change-Id: Ifbd78ad158ae87670bdee4e247469091efaa3260