In two recent commits we fixed the way .message class attribute was
used by passing it as an argument to __init__() of the base Exception
class instead.
Unfortunately, this currently breaks Heat unit tests on Py3K as
exception instances no longer provide .message attribute there.
Re-introduce .message properties only for exception classes that used
to have them to ensure the new oslo.db release do not change the
interface of exceptions.
Closes-Bug: #1542961
Change-Id: I5cae408555956d77e6cbb4e5e513c1c6c375d29d