Fix the explanations of HTTPNotFound for new APIs

Nova wsgi framework gets "explanation" from HTTPNotFound exception and
outputs it into a response body as "message" attribute. Now some error
explanations are passed to HTTPNotFound exception without explanation
argument and they are not output to a response body. As the result,
the default message "The resource could not be found." of wsgi library
are output to a response body instead.

This patch fixes them to output right message to a response body.

The commit I80d3a2da6258e94c485ce024eb310a63bcf39773 has fixed this
kind of bug already, but it cannot cover the ones of new APIs. Then
this patch is necessary again.

Change-Id: Ia784e09cda986a421e83c2820dfd0a2267867cdd
Closes-Bug: #1285437
Merge "Fix the explanations of HTTPNotFound for new APIs"