Print a useful message for unknown server errors

If a server error is returned that has an unknown code, novaclient will
end up printing something similar to:

ClientException: <attribute 'message' of 'exceptions.BaseException' objects>

Setting a message for ClientException will ensure that something more
useful than that is printed.

Change-Id: I43a2a33017f9a5c1b79d7fd8af4153e91d296f7b
Closes-bug: 1295293
Merge "Print a useful message for unknown server errors"