Fix error status code for multinic

When passing bad body in a request, most APIs return BadRequest response.
However, multinic API doesn't do it.
This patch fixes the error status code and adds a unit test related to
this change.

This patch's idea most came from
following patch: https://review.openstack.org/#/c/107266/

Change-Id: If1ef2077a0e9823dfc04e4ef5a6dfb02daf1a8dc
Merge "Fix error status code for multinic"