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

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

changelog.d/receiverworker-error-handling.fix

@@ -0,0 +1 @@
+ReceiverWorker: Make sure non-{:ok, _} is returned as {:error, …}

lib/pleroma/workers/receiver_worker.ex

@@ -52,7 +52,8 @@ defp process_errors(errors) do
       {:error, {:reject, reason}} -> {:cancel, reason}
       {:signature, false} -> {:cancel, :invalid_signature}
       {:error, {:error, reason = "Object has been deleted"}} -> {:cancel, reason}
-      e -> e
+      {:error, _} = e -> e
+      e -> {:error, e}
     end
   end
 end