nat: fixed return values of enable/disable call

NAT44 enable/disable return status was used
instead of appropriate VNET_API_ERROR_ code.

Type: fix
Signed-off-by: Filip Varga <filipvarga89@gmail.com>
Change-Id: If944866bf3061afdc91284c0ad475135e529bdc4
diff --git a/src/vnet/error.h b/src/vnet/error.h
index 39a609b..3feb8ff 100644
--- a/src/vnet/error.h
+++ b/src/vnet/error.h
@@ -156,7 +156,9 @@
   _ (EAGAIN, -165, "Retry stream call with cursor")                           \
   _ (INVALID_VALUE_4, -166, "Invalid value #4")                               \
   _ (BUSY, -167, "Busy")                                                      \
-  _ (BUG, -168, "Bug")
+  _ (BUG, -168, "Bug")                                                        \
+  _ (FEATURE_ALREADY_DISABLED, -169, "Feature already disabled")              \
+  _ (FEATURE_ALREADY_ENABLED, -170, "Feature already enabled")
 
 typedef enum
 {