New feature: create a failing refund

This is currently not working. #459 is tracking it.

features/refunds.feature

@@ -8,6 +8,20 @@ Feature: Refunds
     Then I should get a 201 Created status code
     And the response is valid according to the "refunds" schema
 
+  @failing @gh-459
+  Scenario: Create a failed refund
+    Given I have tokenized a customer card
+    And I have debited that card
+
+    When I make a POST request to the link "debits.refunds" with the body:
+      """
+      {
+        "amount": 200000000
+      }
+      """
+    Then I should get a 400 status code
+    And the response is valid according to the "errors" schema
+
   Scenario: Refund after changed source
     Given I have created a debit
     When I change the default funding source