vnet: introduce vnet_error()

Decouples vnet return values from API return codes.

New vnet_error() creates vnet_error_t whicgh contains both vnet function
return value and return string.

vnet_api_error() converts vlib_error_t constructed with vnet_error() to
API return value.

Type: improvement
Change-Id: I17042954d48c010150fc1dfc5fce9330e8149e87
Signed-off-by: Damjan Marion <damarion@cisco.com>
diff --git a/MAINTAINERS b/MAINTAINERS
index b259754..e89f1aa 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -28,6 +28,11 @@
 F:	build-root/Makefile
 F:	build-data/
 
+VNET
+I:	vnet
+M:	Damjan Marion <damarion@cisco.com>
+F:	src/vnet/error.*
+
 VNET Link Bonding
 I:	bonding
 M:	Steven Luong <sluong@cisco.com>