Add GetRetVal fn to ResponseMessage interface

[sknat]

This patch adds a GetRetVal function on all reply messages, that retreives the error value VPP returns in its Reply. If the field is not present or the value is 0 (success) it returns nil.

Signed-off-by: Nathan Skrzypczak [email protected] Change-Id: I55ca4d02b19909a3fd44c152962b934621646e9f