Fix flash error issues in checkout requests

[Matt-Yorkley]

What? Why?

Closes #5895

Flash error messages created during checkout submission were being persisted in the flash session in unexpected ways, e.g. showing a checkout error message again after navigating away from the checkout page, or showing a message twice.

This was due to saving the errors in the flash session in XHR requests. The session is usually cleared after the subsequent action picks up the flash message, but here we needed to clear it after the current action, as the error is shown immediately via Angular on the checkout page (and shouldn't be persisted after that).

What should we test?

• Using different test cards with Stripe Connect that should give different errors shows the flash errors as expected, instead of showing the previous error message.
• Submitting the checkout with a test card that generates an error message and then navigating to the home page does not re-display the flash error on the home page.

Release notes

Improved flash message display on checkout

[Matt-Yorkley]

I've tried writing controller tests for this, but the Capybara flash object doesn't seem to behave as expected. We need to check what it looks like after the action has finished, but it doesn't seem to be updated within the controller test. I've tested this in the browser and it's working as intended.

[luisramos0]

Nice one!

[mkllnk]

Yes, man! 💐

[filipefurtad0]

Awesome @Matt-Yorkley !

I tested the two scenarios described in the issue and on your test notes:

1. Using different test cards with Stripe Connect that should give different errors shows the flash errors as expected, instead of showing the previous error message. - all good now: the displayed messages correspond to the used cards, as in the video below:

2. Submitting the checkout with a test card that generates an error message and then navigating to the home page does not re-display the flash error on the home page. - no error is seen on the homepage, after triggering an error on checkout.

Good to go, thanks for fixing this! 💪

PS: Outside of the scope of this PR, and to be addressed in a separate issue: - I noticed the case in which a memory-effect is visible after correcting the stock in an order, after reduction on checkout as described here still occurs; also, after triggering this flash message and navigating to the homepage the insufficient stock error appears there as well:

[luisramos0]

Nice finding Filipe, would be good to get an issue for that last point. We will need a similar PR to this one but on another controller 👌