Fix HTTP response code for network APIs and improve error message

This patch corrects the http response codes for network resources to be
consistant with other resources by returning a 400 instead of a 404
if the desired resource is not found.

This patch makes the following api changes:

v2-api: NetworkNotFound now returns 400 instead of 404 and improved error
        message which network was not found instead of
        'The resource could not be found'.

v3-api: PortNotFound and NetworkNotFound now returns a 400 instead of a 404.

Closes-bug: #1245696
DocImpact

Change-Id: I66eb0c0ab926e0a8d1e2c9cfe1f7fd579ea3da21
Merge "Fix HTTP response code for network APIs and improve error message"