Fix the explanations of HTTPNotFound response

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.

Change-Id: I80d3a2da6258e94c485ce024eb310a63bcf39773
Closes-Bug: #1285437
Merge "Fix the explanations of HTTPNotFound response"