Enhance exception translation to better handle NeutronExceptions

Enhance exception translation to better handle NeutronExceptions

NeutronExceptions have a 'message' class attribute that holds the
generic error message template, e.g. "Network %(network)s not found",
unfortunately, because the names are the same, it was overshadowing the
actual exception instance 'message', e.g. "Network 1 not found", after
translation when the exception was serialized to JSON.

This patch puts the exception's actual message in a new field called
'msg' and overwrites NeutronException unicode() so that 'msg' is used
during serialization and we'll get the correct message on the REST API
response.

Fixes bug: #1212882

Change-Id: I3965bffb1c2c2eee0af440d1ecd30ccb3bb958d5