Record invalid payment states for debugging

We observed invalid payment states in Bugsnag but we don't actually know
in which state the payment intent was in. From the context we can guess
that it was "succeeded" but it would be good to validate this. And in
the future it would be good to know if there are other invalid states we
can end up in.

The notification to Bugsnag happens in another part of the code.

spec/models/spree/gateway/stripe_sca_spec.rb

@@ -39,6 +39,16 @@ describe Spree::Gateway::StripeSCA, type: :model do
 
       expect(response.success?).to eq true
     end
+
+    it "provides an error message to help developer debug" do
+      stub_request(:get, "https://api.stripe.com/v1/payment_intents/12345").
+        to_return(status: 200, body: capture_successful)
+
+      response = subject.purchase(order.total, credit_card, gateway_options)
+
+      expect(response.success?).to eq false
+      expect(response.message).to eq "Invalid payment state: succeeded"
+    end
   end
 
   def payment_intent(amount, status)