ReceiverWorker: Make sure non-{:ok, _} is returned as {:error, …}

Otherwise an error like `{:signature, {:error, {:error, :not_found}}}`
ends up considered a success.

Cherry-picked-from: a299ddb10e

lib/pleroma/workers/receiver_worker.ex

@@ -14,7 +14,8 @@ defmodule Pleroma.Workers.ReceiverWorker do
     else
       {:error, :origin_containment_failed} -> {:discard, :origin_containment_failed}
       {:error, {:reject, reason}} -> {:discard, reason}
-      e -> e
+      {:error, _} = e -> e
+      e -> {:error, e}
     end
   end
 end