Improve route error handling in admin/plugins (#18911)

Previously if a specific plugin route was not available (e.g. there was an error loading the plugin's JS due to an ad blocker), the entire page would fail to load. This commit updates the behavior to catch this kind of issue and display a user-friendly message at the top of the screen.

app/assets/javascripts/admin/addon/controllers/admin-plugins.js

@@ -1,17 +1,27 @@
 import { action } from "@ember/object";
 import Controller from "@ember/controller";
-import discourseComputed from "discourse-common/utils/decorators";
+import { inject as service } from "@ember/service";
 
 export default Controller.extend({
-  @discourseComputed
-  adminRoutes() {
+  router: service(),
+
+  get adminRoutes() {
+    return this.allAdminRoutes.filter((r) => this.routeExists(r.full_location));
+  },
+
+  get brokenAdminRoutes() {
+    return this.allAdminRoutes.filter(
+      (r) => !this.routeExists(r.full_location)
+    );
+  },
+
+  get allAdminRoutes() {
     return this.model
+      .filter((p) => p?.enabled)
       .map((p) => {
-        if (p.get("enabled")) {
-          return p.admin_route;
-        }
+        return p.admin_route;
       })
-      .compact();
+      .filter(Boolean);
   },
 
   @action
@@ -21,4 +31,13 @@ export default Controller.extend({
       adminDetail.classList.toggle(state);
     });
   },
+
+  routeExists(routeName) {
+    try {
+      this.router.urlFor(routeName);
+      return true;
+    } catch (e) {
+      return false;
+    }
+  },
 });