Make Ethernet/VLAN Interfaces handle "404 Not Found" properly

Ethernet and VLAN Interfaces lacked detection whether interface
actually existed. So this detection had to be added. Additionally
there was slight inconsistency with error messages and statuses,
this is fixed now.

Tested on x86 development machine and dedicated platform. Does not
introduce any regression.

Change-Id: Ic9e2a98c748ac0dd1cf412b6a54a14f3f53eb9fa
Signed-off-by: Kowalski, Kamil <kamil.kowalski@intel.com>
Signed-off-by: Ed Tanous <ed.tanous@intel.com>