Fix translation of CinderExceptions in REST API

Fix translation of CinderExceptions in REST API

When creating a Fault from a CinderException wrapped in an
HTTPException, we were converting the inner explanation to unicode
before it was able to reach the Fault's call() method which is where
translation occurs, and unicode objects can't be translated.

This patch preserves the CinderException's Message object and puts it in
another Message object as the explanation to the HTTPException so it can
be translated.

Fixes bug: #1229967

Change-Id: Ida71908b639da32b4b85846a117ef21da2fe685b