wireguard: fix error type for crypto backend
wireguard: fix error type for crypto backend

Crypto backend errors should not be using the same error as missing
keypair.

Type: fix

Change-Id: I78c2b3df3f08a354463b7824349b08627f2b023c
Signed-off-by: Benoît Ganne <38aad8ffcfa407b890d258e568f161530f66882d@cisco.com>

FDio/vpp,FDio/vpp,FDio/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,FDio/vpp,chrisy/vpp