Fix: NovaException's message variable should be private

Different exceptions define message as a parameterized string that
cannot be used directly. The complete message with variables replaced
with their real values are only available through the use of str() or
unicode() on the exception class itself.

As such, NovaException's message variable should be renamed _message
to indicate that it is private.

Resolves bug 1154522.

Change-Id: Ibda79de0ac2f94b003fded59c943edb223aeffb6