Fix inconsistent status codes

HTTP 401 should be used when logging in (i.e. authenticating) would make
a difference; HTTP 403 is reserved for requests that fail because the
already authenticated user is not authorized (i.e. lacking permissions)
to do something.

src/Group/Command/CreateGroupHandler.php

@@ -49,6 +49,7 @@ class CreateGroupHandler
         $actor = $command->actor;
         $data = $command->data;
 
+        $this->assertRegistered($actor);
         $this->assertCan($actor, 'createGroup');
 
         $group = Group::build(