Ensure all REST API error responses are consistent

Ensure all REST API error responses are consistent

Heat exceptions are serialized into JSON/XML responses by the fault app,
which is at the end of the WSGI pipeline. Some REST API errors however
are raised as HTTPExceptions, which are treated by WSGI as responses
ready to be sent back to the user, and they can't reach the fault app.

This patch set disguises HTTPExceptions raised by the wsgi.Resource so
they can reach the app that will serialize them just like all other
errors.

Fixes bug 1208620

Change-Id: Id98dbc1e3b208401be8f0119a6d72d4561a2221a