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

test/pleroma/object/fetcher_test.exs

@@ -220,14 +220,14 @@ defmodule Pleroma.Object.FetcherTest do
     end
 
     test "handle HTTP 410 Gone response" do
-      assert {:error, "Object has been deleted"} ==
+      assert {:error, :not_found} ==
                Fetcher.fetch_and_contain_remote_object_from_id(
                  "https://mastodon.example.org/users/userisgone"
                )
     end
 
     test "handle HTTP 404 response" do
-      assert {:error, "Object has been deleted"} ==
+      assert {:error, :not_found} ==
                Fetcher.fetch_and_contain_remote_object_from_id(
                  "https://mastodon.example.org/users/userisgone404"
                )

test/pleroma/web/twitter_api/remote_follow_controller_test.exs

@@ -137,7 +137,7 @@ defmodule Pleroma.Web.TwitterAPI.RemoteFollowControllerTest do
                  |> html_response(200)
 
                assert response =~ "Error fetching user"
-             end) =~ "Object has been deleted"
+             end) =~ ":not_found"
     end
   end