vnet: introduce vnet_error()

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>