vtgate client: minor behavior change for rollback

Rollback should not return an error if we're not in a
transaction. Many frameworks generally perform a rollback
for good measure. So, returning an error just causes confusion.

go/vt/vtgate/gorpcvtgateconn/conn.go

@@ -164,7 +164,7 @@ func (tx *vtgateTx) Commit(ctx context.Context) error {
 
 func (tx *vtgateTx) Rollback(ctx context.Context) error {
 	if tx.session == nil {
-		return errors.New("rollback: not in transaction")
+		return nil
 	}
 	defer func() { tx.session = nil }()
 	return tx.conn.rpcConn.Call(ctx, "VTGate.Rollback", tx.session, &rpc.Unused{})

go/vt/vtgate/vtgateconntest/client.go

@@ -328,9 +328,8 @@ func testTxFail(t *testing.T, conn vtgateconn.VTGateConn) {
 	}
 
 	err = tx.Rollback(ctx)
-	want = "rollback: not in transaction"
-	if err == nil || err.Error() != want {
-		t.Errorf("Rollback: %v, want %v", err, want)
+	if err != nil {
+		t.Error(err)
 	}
 
 	tx, err = conn.Begin(ctx)