adding missing failed states for the NAT Gateways

builtin/providers/aws/resource_aws_nat_gateway.go

@@ -101,7 +101,7 @@ func resourceAwsNatGatewayRead(d *schema.ResourceData, meta interface{}) error {
 	if err != nil {
 		return err
 	}
-	if ngRaw == nil || strings.ToLower(state) == "deleted" {
+	if ngRaw == nil || strings.ToLower(state) == "deleted" || strings.ToLower(state) == "deleting" || strings.ToLower(state) == "failed" {
 		log.Printf("[INFO] Removing %s from Terraform state as it is not found or in the deleted state.", d.Id())
 		d.SetId("")
 		return nil

builtin/providers/aws/resource_aws_nat_gateway_test.go

@@ -44,7 +44,7 @@ func testAccCheckNatGatewayDestroy(s *terraform.State) error {
 			NatGatewayIds: []*string{aws.String(rs.Primary.ID)},
 		})
 		if err == nil {
-			if len(resp.NatGateways) > 0 && strings.ToLower(*resp.NatGateways[0].State) != "deleted" {
+			if len(resp.NatGateways) > 0 && strings.ToLower(*resp.NatGateways[0].State) != "deleted" && strings.ToLower(*resp.NatGateways[0].State) != "deleting" && strings.ToLower(*resp.NatGateways[0].State) != "failed" {
 				return fmt.Errorf("still exists")
 			}