Fail if failover set when set_identifier unset

Added as per
https://github.com/hashicorp/terraform/pull/5777#issuecomment-199802514

builtin/providers/aws/resource_aws_route53_record.go

@@ -455,6 +455,9 @@ func resourceAwsRoute53RecordBuildSet(d *schema.ResourceData, zoneName string) (
 	}
 
 	if v, ok := d.GetOk("failover"); ok {
+		if _, ok := d.GetOk("set_identifier"); !ok {
+			return nil, fmt.Errorf(`provider.aws: aws_route53_record: %s: "set_identifier": required field is not set when "failover" is set`, d.Get("name").(string))
+		}
 		rec.Failover = aws.String(v.(string))
 	}