Policies Error Route (#23516)

* adds error route for policies

* adds changelog entry

changelog/23516.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+ui: Fixes issue with sidebar navigation links disappearing when navigating to policies when a user is not authorized
+```

ui/app/templates/vault/cluster/policies/error.hbs

@@ -0,0 +1,28 @@
+{{!
+  Copyright (c) HashiCorp, Inc.
+  SPDX-License-Identifier: BUSL-1.1
+~}}
+
+{{#if (eq this.model.httpStatus 404)}}
+  <NotFound @model={{this.model}} />
+{{else}}
+  <PageHeader as |p|>
+    <p.levelLeft>
+      <h1 class="title is-3 has-text-grey">
+        {{#if (eq this.model.httpStatus 403)}}
+          Not authorized
+        {{else}}
+          Error
+        {{/if}}
+      </h1>
+    </p.levelLeft>
+  </PageHeader>
+  <div class="box is-sideless has-background-white-bis has-text-grey has-text-centered">
+    {{#if this.model.message}}
+      <p>{{this.model.message}}</p>
+    {{/if}}
+    {{#each this.model.errors as |error|}}
+      <p>{{error}}</p>
+    {{/each}}
+  </div>
+{{/if}}