veyron2/security: Return details on why blessings fail validation.
MultiPart: 6/7
https://github.com/veyron/release-issues/issues/942

Change-Id: I2f38adeab65dd29a4896fa20df70fad24c77a833
diff --git a/go/src/playground/testdata/src/pong/pong.go b/go/src/playground/testdata/src/pong/pong.go
index e89d783..965c3c6 100644
--- a/go/src/playground/testdata/src/pong/pong.go
+++ b/go/src/playground/testdata/src/pong/pong.go
@@ -17,7 +17,7 @@
 type pongd struct{}
 
 func (f *pongd) Ping(ctx ipc.ServerContext, message string) (result string, err error) {
-	remote := ctx.RemoteBlessings().ForContext(ctx)
+	remote, _ := ctx.RemoteBlessings().ForContext(ctx)
 	fmt.Printf("%v: %q\n", remote, message)
 	return "PONG", nil
 }