restapi: Assure network existence when adding a vnic profile

In order to create a vnic profile, the user must provide
the network id to which he wishes to add the profile.
If a network cannot be obtained by the id, the parameters
should consider invalid and a proper message should be
returned.

The backend has logic for this scenario, however when performing
this action with a user that its permissions should be verified,
there is a generic "User is not authorized to perform this action"
even though the message refers to a non-existing network entity.

Change-Id: Ib517cc0fd04c4f0e12fd6bdbdca8c6277600e325
Bug-Url: https://bugzilla.redhat.com/1031369
Signed-off-by: Moti Asayag <masayag@redhat.com>