Handle Phortune charge failures cleanly

Summary:
Ref T2787. Currently, we kill a cart and dead-end the workflow on a charge failure.

Instead, fail the charge and reset the cart so the user can try using a valid payment instrument like a normal checkout workflow would.

Some shakiness/smoothing on WePay for the moment; PayPal is still made up since we don't have a "Hold" state yet.

Test Plan: {F215214}

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2787

Differential Revision: https://secure.phabricator.com/D10666

src/applications/base/controller/PhabricatorController.php

@@ -518,7 +518,7 @@ abstract class PhabricatorController extends AphrontController {
    *
    * @return AphrontDialogView New dialog.
    */
-  protected function newDialog() {
+  public function newDialog() {
     $submit_uri = new PhutilURI($this->getRequest()->getRequestURI());
     $submit_uri = $submit_uri->getPath();