Leverage existing atoms as return errors for the object fetcher

lib/pleroma/object/fetcher.ex

@@ -220,10 +220,10 @@ defmodule Pleroma.Object.Fetcher do
         end
 
       {:ok, %{status: 403}} ->
-        {:error, "Object fetch has been denied"}
+        {:error, :forbidden}
 
       {:ok, %{status: code}} when code in [404, 410] ->
-        {:error, "Object has been deleted"}
+        {:error, :not_found}
 
       {:error, e} ->
         {:error, e}

lib/pleroma/workers/remote_fetcher_worker.ex

@@ -15,11 +15,11 @@ defmodule Pleroma.Workers.RemoteFetcherWorker do
         {:ok, _object} ->
           :ok
 
-        {:error, reason = "Object fetch has been denied"} ->
-          {:cancel, reason}
+        {:error, :forbidden} ->
+          {:cancel, :forbidden}
 
-        {:error, reason = "Object has been deleted"} ->
-          {:cancel, reason}
+        {:error, :not_found} ->
+          {:cancel, :not_found}
 
         _ ->
           :error