feat: add clear authorization error messages for invites

Signed-off-by: Miguel Nogueira <me@nogueira.codes>

app/Http/Requests/ApproveInviteRequest.php

@@ -3,6 +3,7 @@
 namespace App\Http\Requests;
 
 use App\Facades\Options;
+use Illuminate\Auth\Access\AuthorizationException;
 use Illuminate\Foundation\Http\FormRequest;
 
 class ApproveInviteRequest extends FormRequest
@@ -24,4 +25,9 @@ class ApproveInviteRequest extends FormRequest
     {
         return [];
     }
+
+    protected function failedAuthorization()
+    {
+        throw new AuthorizationException(__('Sign ups are open; you can\'t modify invites until you close them.'));
+    }
 }