networkd: show wireguard private key read error number

Noticed while looking at #35641

(cherry picked from commit 0543b02)
(cherry picked from commit b5dc192)
(cherry picked from commit d70b87a)
(cherry picked from commit 549bf6f)
(cherry picked from commit 7fe7bec)
(cherry picked from commit 75ec2e5)

Author: poettering

src/network/netdev/wireguard.c

@@ -1127,7 +1127,7 @@ static int wireguard_verify(NetDev *netdev, const char *filename) {
         r = wireguard_read_key_file(w->private_key_file, w->private_key);
         if (r < 0)
                 return log_netdev_error_errno(netdev, r,
-                                              "Failed to read private key from %s. Ignoring network device.",
+                                              "Failed to read private key from '%s', ignoring network device: %m",
                                               w->private_key_file);
 
         if (eqzero(w->private_key))