Improve /auth/ behavior when a provider implementation is missing

Summary: Ref T1536. This "should never happen", but can if you're developing custom providers. Improve the robustness of this interface in the presence of missing provider implementations.

Test Plan: {F47008}

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6253

src/applications/auth/controller/config/PhabricatorAuthListController.php

@@ -25,17 +25,25 @@ final class PhabricatorAuthListController
       if ($provider) {
         $name = $provider->getProviderName();
       } else {
-        $name = $config->getProviderType();
+        $name = $config->getProviderType().' ('.$config->getProviderClass().')';
       }
 
       $item
-        ->setHeader($name)
+        ->setHeader($name);
-        ->setHref($edit_uri);
 
+      if ($provider) {
+        $item->setHref($edit_uri);
+      } else {
+        $item->addAttribute(pht('Provider Implementation Missing!'));
+      }
+
+      $domain = null;
+      if ($provider) {
         $domain = $provider->getProviderDomain();
         if ($domain !== 'self') {
           $item->addAttribute($domain);
         }
+      }
 
       if ($config->getShouldAllowRegistration()) {
         $item->addAttribute(pht('Allows Registration'));