wgengine: fail NewUserspaceEngine if wireguard device doesn't come up

Just something I ran across while debugging an unrelated failure. This
is not in response to any bug/issue.

Signed-off-by: Maisem Ali <maisem@tailscale.com>

wgengine/userspace.go

@@ -388,7 +388,9 @@ func NewUserspaceEngine(logf logger.Logf, conf Config) (_ Engine, reterr error)
 	}()
 
 	e.logf("Bringing wireguard device up...")
-	e.wgdev.Up()
+	if err := e.wgdev.Up(); err != nil {
+		return nil, fmt.Errorf("wgdev.Up(): %w", err)
+	}
 	e.logf("Bringing router up...")
 	if err := e.router.Up(); err != nil {
 		return nil, err