Display notice when accessing deprecated WC_Cart->discount_total

When accessing the `WC()->cart->discount_total` property directly, display a notice and then return `0` (rather than just retuning `0`) to get the information in front of developers that are not tuned into the [develop blog](http://develop.woothemes.com/woocommerce/2014/12/upcoming-coupon-changes-in-woocommerce-2-3/).

Example notice: 

```
PHP Notice:  WC_Cart->discount_total was called with an argument that is <strong>deprecated</strong> since version 2.3! After tax coupons are no longer supported. For more information see: http://develop.woothemes.com/woocommerce/2014/12/upcoming-coupon-changes-in-woocommerce-2-3/ in /wp-includes/functions.php on line 3318
```

Related to #6830.