Lack of user feedback upon use of declined card

Ext version: 2.5.2

Summary: When a user's credit card is declined, the user is redirected to the contribution-page/event-registration main form, without any error message or indication of the problem.

Example steps to reproduce:

1. Create a simple contribution page using this payment processor
2. Open the live the contribution page form and enter details for a credit card which will pass validation but will be declined (I believe it's sufficient to use the usual test credit card number); click "Review your contribution" button.
3. (If the "Use a confirmation page?" setting is enabled for this page: Observe the confirmation page is loaded; click the "Make Contribution" button on this page.)
4. Observe the issue here.

• Expected behavior: there should be some indication that the card was declined.
• Actual behavior: the main contribution page form is loaded (with query parameter reset=1, so fields are empty), and there is no indication that the card was declined.

Other notes:

• We've checked in account.authorize.net to verify that the observed bad behavior is happening when an actual credit card decline happens.
• Naturally I can't share (and in fact don't even know) the full details of an actual declined card, and of course couldn't provide those here anyway; but the usual 4111111111111111 card seems to be sufficient for repro.

(Joinery ticket reference: F#920.)