Avoid serializing CinderExceptions before they are translated

Avoid serializing CinderExceptions before they are translated

CinderExceptions were being unicode()'d when being wrapped in an
HTTPException, and this was causing the delayed translation to fail for
those errors.

Also, CinderExceptions have a 'message' class attribute that holds the
generic error message template, e.g. "Backup  %(backup_id)s is not
found", unfortunately, because the names are the same, it was
overshadowing the actual exception instance 'message', e.g. "Backup 1 is
not found", when translating. This patch puts the exception's actual
message in a new field called 'msg'.

Fixes Bug: #1214102

Change-Id: Ied9abcc3d05454852c0a5891432eb181220a744e