* Add functionality to wp-admin order to check for coupon usage based on email addresses, similar to how WC_Cart handles this seperately, included unit tests.
* Only do the coupon by email usage check if order is from a guest. Fix issue with unit test.
This commit adds the exception message thrown when WC_Order::set_billing_email() is called with an invalid message to a test to fix it.
I couldn't find what change in WC codebase broke the test and I'm not sure why it is failing only when running using PHP 5.6:
https://travis-ci.org/woocommerce/woocommerce/jobs/315750242#L600