validators/add_remove: don't crash on failure to resolve reference

It allows for informed error handling and retry/discard job
decisions lateron which a future commit will add.

lib/pleroma/web/activity_pub/object_validators/add_remove_validator.ex

@@ -9,6 +9,7 @@ defmodule Pleroma.Web.ActivityPub.ObjectValidators.AddRemoveValidator do
   import Pleroma.Web.ActivityPub.ObjectValidators.CommonValidations
 
   require Pleroma.Constants
+  require Logger
 
   alias Pleroma.User
 
@@ -27,14 +28,21 @@ defmodule Pleroma.Web.ActivityPub.ObjectValidators.AddRemoveValidator do
   end
 
   def cast_and_validate(data) do
-    {:ok, actor} = User.get_or_fetch_by_ap_id(data["actor"])
+    with {_, {:ok, actor}} <- {:user, User.get_or_fetch_by_ap_id(data["actor"])},
-
+         {_, {:ok, actor}} <- {:feataddr, maybe_refetch_user(actor)} do
-    {:ok, actor} = maybe_refetch_user(actor)
-
       data
       |> maybe_fix_data_for_mastodon(actor)
       |> cast_data()
       |> validate_data(actor)
+    else
+      {:feataddr, _} ->
+        {:error,
+         {:validate,
+          "Actor doesn't provide featured collection address to verify against: #{data["id"]}"}}
+
+      {:user, _} ->
+        {:error, :link_resolve_failed}
+    end
   end
 
   defp maybe_fix_data_for_mastodon(data, actor) do