Send ProviderException with the correct error code and message
This makes a ProviderEexception(400, ..) actually send a 400 status code,
and display the correct message.
Otherwise, we were sending an error 500, because cherrypy did not
know how to interpret Exception's other than cherrypy.HTTPError.
Signed-off-by: Patrick Uiterwijk <puiterwijk@redhat.com>
Reviewed-by: Pierre-Yves Chibon <pingou@pingoured.fr>