Fix checkout flow activity close on error.

app/src/main/java/org/thoughtcrime/securesms/badges/gifts/flow/GiftFlowConfirmationFragment.kt

@@ -313,4 +313,8 @@ class GiftFlowConfirmationFragment :
   override fun onUserLaunchedAnExternalApplication() = error("Not supported for gifts.")
 
   override fun navigateToDonationPending(inAppPayment: InAppPaymentTable.InAppPayment) = error("Not supported for gifts")
+
+  override fun exitCheckoutFlow() {
+    requireActivity().finishAfterTransition()
+  }
 }